[PATCH v3] um: Enable preemption in UML

[PATCH v3] um: Enable preemption in UML

[anton.ivanov]

From: Anton Ivanov <anton.ivanov@cambridgegreys.com>

Preemption requires saving/restoring FPU state. This patch
adds support for it using GCC intrinsics.

Signed-off-by: Anton Ivanov <anton.ivanov@cambridgegreys.com>
---
 arch/um/Kconfig                         |  1 -
 arch/um/include/asm/fpu/api.h           |  6 +-
 arch/um/include/asm/processor-generic.h |  3 +
 arch/um/kernel/Makefile                 |  4 ++
 arch/um/kernel/fpu.c                    | 77 +++++++++++++++++++++++++
 arch/um/kernel/irq.c                    |  2 +
 6 files changed, 91 insertions(+), 2 deletions(-)
 create mode 100644 arch/um/kernel/fpu.c

diff --git a/arch/um/Kconfig b/arch/um/Kconfig
index b5e179360534..603f5fd82293 100644
--- a/arch/um/Kconfig
+++ b/arch/um/Kconfig
@@ -11,7 +11,6 @@ config UML
 	select ARCH_HAS_KCOV
 	select ARCH_HAS_STRNCPY_FROM_USER
 	select ARCH_HAS_STRNLEN_USER
-	select ARCH_NO_PREEMPT
 	select HAVE_ARCH_AUDITSYSCALL
 	select HAVE_ARCH_KASAN if X86_64
 	select HAVE_ARCH_KASAN_VMALLOC if HAVE_ARCH_KASAN
diff --git a/arch/um/include/asm/fpu/api.h b/arch/um/include/asm/fpu/api.h
index 71bfd9ef3938..76ffa9c3c4b9 100644
--- a/arch/um/include/asm/fpu/api.h
+++ b/arch/um/include/asm/fpu/api.h
@@ -7,9 +7,13 @@
  * A set of "dummy" defines to allow the direct inclusion
  * of x86 optimized copy, xor, etc routines into the
  * UML code tree. */
-
+#if defined(CONFIG_PREEMPT) || defined(CONFIG_PREEMPT_VOLUNTRARY)
+extern void kernel_fpu_begin(void);
+extern void kernel_fpu_end(void);
+#else
 #define kernel_fpu_begin() (void)0
 #define kernel_fpu_end() (void)0
+#endif
 
 static inline bool irq_fpu_usable(void)
 {
diff --git a/arch/um/include/asm/processor-generic.h b/arch/um/include/asm/processor-generic.h
index 7414154b8e9a..d9362f5a2212 100644
--- a/arch/um/include/asm/processor-generic.h
+++ b/arch/um/include/asm/processor-generic.h
@@ -44,6 +44,9 @@ struct thread_struct {
 			} cb;
 		} u;
 	} request;
+#if defined(CONFIG_PREEMPT) || defined(CONFIG_PREEMPT_VOLUNTRARY)
+    u8 fpu[2048] __aligned(64); /* Intel docs require xsave/xrestore area to be aligned to 16 bytes */
+#endif
 };
 
 #define INIT_THREAD \
diff --git a/arch/um/kernel/Makefile b/arch/um/kernel/Makefile
index 811188be954c..c616e884a488 100644
--- a/arch/um/kernel/Makefile
+++ b/arch/um/kernel/Makefile
@@ -26,9 +26,13 @@ obj-$(CONFIG_OF) += dtb.o
 obj-$(CONFIG_EARLY_PRINTK) += early_printk.o
 obj-$(CONFIG_STACKTRACE) += stacktrace.o
 obj-$(CONFIG_GENERIC_PCI_IOMAP) += ioport.o
+obj-$(CONFIG_PREEMPT) += fpu.o
+obj-$(CONFIG_PREEMPT_VOLUNTARY) += fpu.o
 
 USER_OBJS := config.o
 
+CFLAGS_fpu.o += -mxsave -mxsaveopt
+
 include $(srctree)/arch/um/scripts/Makefile.rules
 
 targets := config.c config.tmp capflags.c
diff --git a/arch/um/kernel/fpu.c b/arch/um/kernel/fpu.c
new file mode 100644
index 000000000000..346c07236185
--- /dev/null
+++ b/arch/um/kernel/fpu.c
@@ -0,0 +1,77 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (C) 2023 Cambridge Greys Ltd
+ * Copyright (C) 2023 Red Hat Inc
+ */
+
+#include <linux/cpu.h>
+#include <linux/init.h>
+#include <asm/fpu/api.h>
+#include <asm/cpufeature.h>
+
+/*
+ * The critical section between kernel_fpu_begin() and kernel_fpu_end()
+ * is non-reentrant. It is the caller's responsibility to avoid reentrance.
+ */
+
+static DEFINE_PER_CPU(bool, in_kernel_fpu);
+
+/* UML and driver code it pulls out of the x86 tree knows about 387 features
+ * up to and including AVX512. TILE, etc are not yet supported.
+ */
+
+#define KNOWN_387_FEATURES 0xFF
+
+
+void kernel_fpu_begin(void)
+{
+	preempt_disable();
+
+	WARN_ON(this_cpu_read(in_kernel_fpu));
+
+	this_cpu_write(in_kernel_fpu, true);
+
+#ifdef CONFIG_64BIT
+	if (likely(cpu_has(&boot_cpu_data, X86_FEATURE_XSAVEOPT)))
+		__builtin_ia32_xsaveopt64(&current->thread.fpu, KNOWN_387_FEATURES);
+	else {
+		if (likely(cpu_has(&boot_cpu_data, X86_FEATURE_XSAVE)))
+			__builtin_ia32_xsave64(&current->thread.fpu, KNOWN_387_FEATURES);
+		else
+			__builtin_ia32_fxsave64(&current->thread.fpu);
+	}
+#else
+	if (likely(cpu_has(&boot_cpu_data, X86_FEATURE_XSAVEOPT)))
+		__builtin_ia32_xsaveopt(&current->thread.fpu, KNOWN_387_FEATURES);
+	else {
+		if (likely(cpu_has(&boot_cpu_data, X86_FEATURE_XSAVE)))
+			__builtin_ia32_xsave(&current->thread.fpu, KNOWN_387_FEATURES);
+		else
+			__builtin_ia32_fxsave(&current->thread.fpu);
+	}
+#endif
+}
+
+EXPORT_SYMBOL_GPL(kernel_fpu_begin);
+
+void kernel_fpu_end(void)
+{
+	WARN_ON(!this_cpu_read(in_kernel_fpu));
+
+#ifdef CONFIG_64BIT
+	if (likely(cpu_has(&boot_cpu_data, X86_FEATURE_XSAVE)))
+		__builtin_ia32_xrstor64(&current->thread.fpu, KNOWN_387_FEATURES);
+	else
+		__builtin_ia32_fxrstor64(&current->thread.fpu);
+#else
+	if (likely(cpu_has(&boot_cpu_data, X86_FEATURE_XSAVE)))
+		__builtin_ia32_xrstor(&current->thread.fpu, KNOWN_387_FEATURES);
+	else
+		__builtin_ia32_fxrstor(&current->thread.fpu);
+#endif
+	this_cpu_write(in_kernel_fpu, false);
+
+	preempt_enable();
+}
+EXPORT_SYMBOL_GPL(kernel_fpu_end);
+
diff --git a/arch/um/kernel/irq.c b/arch/um/kernel/irq.c
index 635d44606bfe..c02525da45df 100644
--- a/arch/um/kernel/irq.c
+++ b/arch/um/kernel/irq.c
@@ -195,7 +195,9 @@ static void _sigio_handler(struct uml_pt_regs *regs,
 
 void sigio_handler(int sig, struct siginfo *unused_si, struct uml_pt_regs *regs)
 {
+	preempt_disable();
 	_sigio_handler(regs, irqs_suspended);
+	preempt_enable();
 }
 
 static struct irq_entry *get_irq_entry_by_fd(int fd)
-- 
2.30.2


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Re: [PATCH v3] um: Enable preemption in UML

[Anton Ivanov]

On 21/09/2023 16:32, anton.ivanov@cambridgegreys.com wrote:
> From: Anton Ivanov <anton.ivanov@cambridgegreys.com>
> 
> Preemption requires saving/restoring FPU state. This patch
> adds support for it using GCC intrinsics.
> 
> Signed-off-by: Anton Ivanov <anton.ivanov@cambridgegreys.com>
> ---
>   arch/um/Kconfig                         |  1 -
>   arch/um/include/asm/fpu/api.h           |  6 +-
>   arch/um/include/asm/processor-generic.h |  3 +
>   arch/um/kernel/Makefile                 |  4 ++
>   arch/um/kernel/fpu.c                    | 77 +++++++++++++++++++++++++
>   arch/um/kernel/irq.c                    |  2 +
>   6 files changed, 91 insertions(+), 2 deletions(-)
>   create mode 100644 arch/um/kernel/fpu.c


Fixes:
	1. Defines. I messed them up in v2.
	2. Makes build selective on CONFIG_PREEMPT. If preemption is not enabled fpu.o is
not build.
	3. CFLAGS are altered only for fpu.o

Performance change is the same - from 20minutes for find /usr and cat to /dev/null down to 3
and from unusable under stress to fairly reasonable.

> 
> diff --git a/arch/um/Kconfig b/arch/um/Kconfig
> index b5e179360534..603f5fd82293 100644
> --- a/arch/um/Kconfig
> +++ b/arch/um/Kconfig
> @@ -11,7 +11,6 @@ config UML
>   	select ARCH_HAS_KCOV
>   	select ARCH_HAS_STRNCPY_FROM_USER
>   	select ARCH_HAS_STRNLEN_USER
> -	select ARCH_NO_PREEMPT
>   	select HAVE_ARCH_AUDITSYSCALL
>   	select HAVE_ARCH_KASAN if X86_64
>   	select HAVE_ARCH_KASAN_VMALLOC if HAVE_ARCH_KASAN
> diff --git a/arch/um/include/asm/fpu/api.h b/arch/um/include/asm/fpu/api.h
> index 71bfd9ef3938..76ffa9c3c4b9 100644
> --- a/arch/um/include/asm/fpu/api.h
> +++ b/arch/um/include/asm/fpu/api.h
> @@ -7,9 +7,13 @@
>    * A set of "dummy" defines to allow the direct inclusion
>    * of x86 optimized copy, xor, etc routines into the
>    * UML code tree. */
> -
> +#if defined(CONFIG_PREEMPT) || defined(CONFIG_PREEMPT_VOLUNTRARY)
> +extern void kernel_fpu_begin(void);
> +extern void kernel_fpu_end(void);
> +#else
>   #define kernel_fpu_begin() (void)0
>   #define kernel_fpu_end() (void)0
> +#endif
>   
>   static inline bool irq_fpu_usable(void)
>   {
> diff --git a/arch/um/include/asm/processor-generic.h b/arch/um/include/asm/processor-generic.h
> index 7414154b8e9a..d9362f5a2212 100644
> --- a/arch/um/include/asm/processor-generic.h
> +++ b/arch/um/include/asm/processor-generic.h
> @@ -44,6 +44,9 @@ struct thread_struct {
>   			} cb;
>   		} u;
>   	} request;
> +#if defined(CONFIG_PREEMPT) || defined(CONFIG_PREEMPT_VOLUNTRARY)
> +    u8 fpu[2048] __aligned(64); /* Intel docs require xsave/xrestore area to be aligned to 16 bytes */
> +#endif
>   };
>   
>   #define INIT_THREAD \
> diff --git a/arch/um/kernel/Makefile b/arch/um/kernel/Makefile
> index 811188be954c..c616e884a488 100644
> --- a/arch/um/kernel/Makefile
> +++ b/arch/um/kernel/Makefile
> @@ -26,9 +26,13 @@ obj-$(CONFIG_OF) += dtb.o
>   obj-$(CONFIG_EARLY_PRINTK) += early_printk.o
>   obj-$(CONFIG_STACKTRACE) += stacktrace.o
>   obj-$(CONFIG_GENERIC_PCI_IOMAP) += ioport.o
> +obj-$(CONFIG_PREEMPT) += fpu.o
> +obj-$(CONFIG_PREEMPT_VOLUNTARY) += fpu.o
>   
>   USER_OBJS := config.o
>   
> +CFLAGS_fpu.o += -mxsave -mxsaveopt
> +
>   include $(srctree)/arch/um/scripts/Makefile.rules
>   
>   targets := config.c config.tmp capflags.c
> diff --git a/arch/um/kernel/fpu.c b/arch/um/kernel/fpu.c
> new file mode 100644
> index 000000000000..346c07236185
> --- /dev/null
> +++ b/arch/um/kernel/fpu.c
> @@ -0,0 +1,77 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * Copyright (C) 2023 Cambridge Greys Ltd
> + * Copyright (C) 2023 Red Hat Inc
> + */
> +
> +#include <linux/cpu.h>
> +#include <linux/init.h>
> +#include <asm/fpu/api.h>
> +#include <asm/cpufeature.h>
> +
> +/*
> + * The critical section between kernel_fpu_begin() and kernel_fpu_end()
> + * is non-reentrant. It is the caller's responsibility to avoid reentrance.
> + */
> +
> +static DEFINE_PER_CPU(bool, in_kernel_fpu);
> +
> +/* UML and driver code it pulls out of the x86 tree knows about 387 features
> + * up to and including AVX512. TILE, etc are not yet supported.
> + */
> +
> +#define KNOWN_387_FEATURES 0xFF
> +
> +
> +void kernel_fpu_begin(void)
> +{
> +	preempt_disable();
> +
> +	WARN_ON(this_cpu_read(in_kernel_fpu));
> +
> +	this_cpu_write(in_kernel_fpu, true);
> +
> +#ifdef CONFIG_64BIT
> +	if (likely(cpu_has(&boot_cpu_data, X86_FEATURE_XSAVEOPT)))
> +		__builtin_ia32_xsaveopt64(&current->thread.fpu, KNOWN_387_FEATURES);
> +	else {
> +		if (likely(cpu_has(&boot_cpu_data, X86_FEATURE_XSAVE)))
> +			__builtin_ia32_xsave64(&current->thread.fpu, KNOWN_387_FEATURES);
> +		else
> +			__builtin_ia32_fxsave64(&current->thread.fpu);
> +	}
> +#else
> +	if (likely(cpu_has(&boot_cpu_data, X86_FEATURE_XSAVEOPT)))
> +		__builtin_ia32_xsaveopt(&current->thread.fpu, KNOWN_387_FEATURES);
> +	else {
> +		if (likely(cpu_has(&boot_cpu_data, X86_FEATURE_XSAVE)))
> +			__builtin_ia32_xsave(&current->thread.fpu, KNOWN_387_FEATURES);
> +		else
> +			__builtin_ia32_fxsave(&current->thread.fpu);
> +	}
> +#endif
> +}
> +
> +EXPORT_SYMBOL_GPL(kernel_fpu_begin);
> +
> +void kernel_fpu_end(void)
> +{
> +	WARN_ON(!this_cpu_read(in_kernel_fpu));
> +
> +#ifdef CONFIG_64BIT
> +	if (likely(cpu_has(&boot_cpu_data, X86_FEATURE_XSAVE)))
> +		__builtin_ia32_xrstor64(&current->thread.fpu, KNOWN_387_FEATURES);
> +	else
> +		__builtin_ia32_fxrstor64(&current->thread.fpu);
> +#else
> +	if (likely(cpu_has(&boot_cpu_data, X86_FEATURE_XSAVE)))
> +		__builtin_ia32_xrstor(&current->thread.fpu, KNOWN_387_FEATURES);
> +	else
> +		__builtin_ia32_fxrstor(&current->thread.fpu);
> +#endif
> +	this_cpu_write(in_kernel_fpu, false);
> +
> +	preempt_enable();
> +}
> +EXPORT_SYMBOL_GPL(kernel_fpu_end);
> +
> diff --git a/arch/um/kernel/irq.c b/arch/um/kernel/irq.c
> index 635d44606bfe..c02525da45df 100644
> --- a/arch/um/kernel/irq.c
> +++ b/arch/um/kernel/irq.c
> @@ -195,7 +195,9 @@ static void _sigio_handler(struct uml_pt_regs *regs,
>   
>   void sigio_handler(int sig, struct siginfo *unused_si, struct uml_pt_regs *regs)
>   {
> +	preempt_disable();
>   	_sigio_handler(regs, irqs_suspended);
> +	preempt_enable();
>   }
>   
>   static struct irq_entry *get_irq_entry_by_fd(int fd)

-- 
Anton R. Ivanov
Cambridgegreys Limited. Registered in England. Company Number 10273661
https://www.cambridgegreys.com/

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Re: [PATCH v3] um: Enable preemption in UML

[Peter Lafreniere]

On Thu, Sep 21, 2023 at 11:32, anton.ivanov@cambridgegreys.com wrote:

> From: Anton Ivanov anton.ivanov@cambridgegreys.com
> 
> 
> Preemption requires saving/restoring FPU state. This patch
> adds support for it using GCC intrinsics.
> 
> Signed-off-by: Anton Ivanov anton.ivanov@cambridgegreys.com
> 
> ---
> arch/um/Kconfig | 1 -
> arch/um/include/asm/fpu/api.h | 6 +-
> arch/um/include/asm/processor-generic.h | 3 +
> arch/um/kernel/Makefile | 4 ++
> arch/um/kernel/fpu.c | 77 +++++++++++++++++++++++++
> arch/um/kernel/irq.c | 2 +
> 6 files changed, 91 insertions(+), 2 deletions(-)
> create mode 100644 arch/um/kernel/fpu.c
> 
> diff --git a/arch/um/Kconfig b/arch/um/Kconfig
> index b5e179360534..603f5fd82293 100644
> --- a/arch/um/Kconfig
> +++ b/arch/um/Kconfig
> @@ -11,7 +11,6 @@ config UML
> select ARCH_HAS_KCOV
> select ARCH_HAS_STRNCPY_FROM_USER
> select ARCH_HAS_STRNLEN_USER
> - select ARCH_NO_PREEMPT
> select HAVE_ARCH_AUDITSYSCALL
> select HAVE_ARCH_KASAN if X86_64
> select HAVE_ARCH_KASAN_VMALLOC if HAVE_ARCH_KASAN
> diff --git a/arch/um/include/asm/fpu/api.h b/arch/um/include/asm/fpu/api.h
> index 71bfd9ef3938..76ffa9c3c4b9 100644
> --- a/arch/um/include/asm/fpu/api.h
> +++ b/arch/um/include/asm/fpu/api.h
> @@ -7,9 +7,13 @@
> * A set of "dummy" defines to allow the direct inclusion
> * of x86 optimized copy, xor, etc routines into the
> * UML code tree. /
> -
> +#if defined(CONFIG_PREEMPT) || defined(CONFIG_PREEMPT_VOLUNTRARY)

Typo, should be CONFIG_PREEMPT_VOLUNTARY

> +extern void kernel_fpu_begin(void);
> +extern void kernel_fpu_end(void);
> +#else
> #define kernel_fpu_begin() (void)0
> #define kernel_fpu_end() (void)0
> +#endif
> 
> static inline bool irq_fpu_usable(void)
> {
> diff --git a/arch/um/include/asm/processor-generic.h b/arch/um/include/asm/processor-generic.h
> index 7414154b8e9a..d9362f5a2212 100644
> --- a/arch/um/include/asm/processor-generic.h
> +++ b/arch/um/include/asm/processor-generic.h
> @@ -44,6 +44,9 @@ struct thread_struct {
> } cb;
> } u;
> } request;
> +#if defined(CONFIG_PREEMPT) || defined(CONFIG_PREEMPT_VOLUNTRARY)

Same typo again.

> + u8 fpu[2048] __aligned(64); / Intel docs require xsave/xrestore area to be aligned to 16 bytes /
> +#endif
> };
> 
> #define INIT_THREAD \
> diff --git a/arch/um/kernel/Makefile b/arch/um/kernel/Makefile
> index 811188be954c..c616e884a488 100644
> --- a/arch/um/kernel/Makefile
> +++ b/arch/um/kernel/Makefile
> @@ -26,9 +26,13 @@ obj-$(CONFIG_OF) += dtb.o
> obj-$(CONFIG_EARLY_PRINTK) += early_printk.o
> obj-$(CONFIG_STACKTRACE) += stacktrace.o
> obj-$(CONFIG_GENERIC_PCI_IOMAP) += ioport.o
> +obj-$(CONFIG_PREEMPT) += fpu.o
> +obj-$(CONFIG_PREEMPT_VOLUNTARY) += fpu.o
> 
> USER_OBJS := config.o
> 
> +CFLAGS_fpu.o += -mxsave -mxsaveopt
> +
> include $(srctree)/arch/um/scripts/Makefile.rules
> 
> targets := config.c config.tmp capflags.c
> diff --git a/arch/um/kernel/fpu.c b/arch/um/kernel/fpu.c
> new file mode 100644
> index 000000000000..346c07236185
> --- /dev/null
> +++ b/arch/um/kernel/fpu.c
> @@ -0,0 +1,77 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/
> + * Copyright (C) 2023 Cambridge Greys Ltd
> + * Copyright (C) 2023 Red Hat Inc
> + */
> +
> +#include <linux/cpu.h>
> 
> +#include <linux/init.h>
> 
> +#include <asm/fpu/api.h>
> 
> +#include <asm/cpufeature.h>
> 
> +
> +/*
> + * The critical section between kernel_fpu_begin() and kernel_fpu_end()
> + * is non-reentrant. It is the caller's responsibility to avoid reentrance.
> + /
> +
> +static DEFINE_PER_CPU(bool, in_kernel_fpu);
> +
> +/ UML and driver code it pulls out of the x86 tree knows about 387 features
> + * up to and including AVX512. TILE, etc are not yet supported.
> + */
> +
> +#define KNOWN_387_FEATURES 0xFF
> +
> +
> +void kernel_fpu_begin(void)
> +{
> + preempt_disable();
> +
> + WARN_ON(this_cpu_read(in_kernel_fpu));
> +
> + this_cpu_write(in_kernel_fpu, true);
> +
> +#ifdef CONFIG_64BIT
> + if (likely(cpu_has(&boot_cpu_data, X86_FEATURE_XSAVEOPT)))
> + __builtin_ia32_xsaveopt64(&current->thread.fpu, KNOWN_387_FEATURES);
> 
> + else {
> + if (likely(cpu_has(&boot_cpu_data, X86_FEATURE_XSAVE)))
> + __builtin_ia32_xsave64(&current->thread.fpu, KNOWN_387_FEATURES);
> 
> + else
> + __builtin_ia32_fxsave64(&current->thread.fpu);
> 
> + }
> +#else
> + if (likely(cpu_has(&boot_cpu_data, X86_FEATURE_XSAVEOPT)))
> + __builtin_ia32_xsaveopt(&current->thread.fpu, KNOWN_387_FEATURES);
> 
> + else {
> + if (likely(cpu_has(&boot_cpu_data, X86_FEATURE_XSAVE)))
> + __builtin_ia32_xsave(&current->thread.fpu, KNOWN_387_FEATURES);
> 
> + else
> + __builtin_ia32_fxsave(&current->thread.fpu);
> 
> + }
> +#endif
> +}
> +
> +EXPORT_SYMBOL_GPL(kernel_fpu_begin);
> +
> +void kernel_fpu_end(void)
> +{
> + WARN_ON(!this_cpu_read(in_kernel_fpu));
> +
> +#ifdef CONFIG_64BIT
> + if (likely(cpu_has(&boot_cpu_data, X86_FEATURE_XSAVE)))
> + __builtin_ia32_xrstor64(&current->thread.fpu, KNOWN_387_FEATURES);
> 
> + else
> + __builtin_ia32_fxrstor64(&current->thread.fpu);
> 
> +#else
> + if (likely(cpu_has(&boot_cpu_data, X86_FEATURE_XSAVE)))
> + __builtin_ia32_xrstor(&current->thread.fpu, KNOWN_387_FEATURES);
> 
> + else
> + __builtin_ia32_fxrstor(&current->thread.fpu);
> 
> +#endif
> + this_cpu_write(in_kernel_fpu, false);
> +
> + preempt_enable();
> +}
> +EXPORT_SYMBOL_GPL(kernel_fpu_end);
> +
> diff --git a/arch/um/kernel/irq.c b/arch/um/kernel/irq.c
> index 635d44606bfe..c02525da45df 100644
> --- a/arch/um/kernel/irq.c
> +++ b/arch/um/kernel/irq.c
> @@ -195,7 +195,9 @@ static void _sigio_handler(struct uml_pt_regs *regs,
> 
> void sigio_handler(int sig, struct siginfo *unused_si, struct uml_pt_regs *regs)
> {
> + preempt_disable();
> _sigio_handler(regs, irqs_suspended);
> + preempt_enable();
> }
> 
> static struct irq_entry *get_irq_entry_by_fd(int fd)

Everything other than the typos look good to me. I still think that there
should be some limits on reflection, at least when fxsave is being used
(when running on a poorly-confgured VM with AVX and no xsave, for example).

Cheers,

Peter Lafreniere
<peter@n8pjl.ca>


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Re: [PATCH v3] um: Enable preemption in UML

[Anton Ivanov]

On 21/09/2023 16:42, Peter Lafreniere wrote:
> On Thu, Sep 21, 2023 at 11:32, anton.ivanov@cambridgegreys.com wrote:
>
>> From: Anton Ivanov anton.ivanov@cambridgegreys.com
>>
>>
>> Preemption requires saving/restoring FPU state. This patch
>> adds support for it using GCC intrinsics.
>>
>> Signed-off-by: Anton Ivanov anton.ivanov@cambridgegreys.com
>>
>> ---
>> arch/um/Kconfig | 1 -
>> arch/um/include/asm/fpu/api.h | 6 +-
>> arch/um/include/asm/processor-generic.h | 3 +
>> arch/um/kernel/Makefile | 4 ++
>> arch/um/kernel/fpu.c | 77 +++++++++++++++++++++++++
>> arch/um/kernel/irq.c | 2 +
>> 6 files changed, 91 insertions(+), 2 deletions(-)
>> create mode 100644 arch/um/kernel/fpu.c
>>
>> diff --git a/arch/um/Kconfig b/arch/um/Kconfig
>> index b5e179360534..603f5fd82293 100644
>> --- a/arch/um/Kconfig
>> +++ b/arch/um/Kconfig
>> @@ -11,7 +11,6 @@ config UML
>> select ARCH_HAS_KCOV
>> select ARCH_HAS_STRNCPY_FROM_USER
>> select ARCH_HAS_STRNLEN_USER
>> - select ARCH_NO_PREEMPT
>> select HAVE_ARCH_AUDITSYSCALL
>> select HAVE_ARCH_KASAN if X86_64
>> select HAVE_ARCH_KASAN_VMALLOC if HAVE_ARCH_KASAN
>> diff --git a/arch/um/include/asm/fpu/api.h b/arch/um/include/asm/fpu/api.h
>> index 71bfd9ef3938..76ffa9c3c4b9 100644
>> --- a/arch/um/include/asm/fpu/api.h
>> +++ b/arch/um/include/asm/fpu/api.h
>> @@ -7,9 +7,13 @@
>> * A set of "dummy" defines to allow the direct inclusion
>> * of x86 optimized copy, xor, etc routines into the
>> * UML code tree. /
>> -
>> +#if defined(CONFIG_PREEMPT) || defined(CONFIG_PREEMPT_VOLUNTRARY)
> Typo, should be CONFIG_PREEMPT_VOLUNTARY

Guilty as charged. Fixing it.

>
>> +extern void kernel_fpu_begin(void);
>> +extern void kernel_fpu_end(void);
>> +#else
>> #define kernel_fpu_begin() (void)0
>> #define kernel_fpu_end() (void)0
>> +#endif
>>
>> static inline bool irq_fpu_usable(void)
>> {
>> diff --git a/arch/um/include/asm/processor-generic.h b/arch/um/include/asm/processor-generic.h
>> index 7414154b8e9a..d9362f5a2212 100644
>> --- a/arch/um/include/asm/processor-generic.h
>> +++ b/arch/um/include/asm/processor-generic.h
>> @@ -44,6 +44,9 @@ struct thread_struct {
>> } cb;
>> } u;
>> } request;
>> +#if defined(CONFIG_PREEMPT) || defined(CONFIG_PREEMPT_VOLUNTRARY)
> Same typo again.
>
>> + u8 fpu[2048] __aligned(64); / Intel docs require xsave/xrestore area to be aligned to 16 bytes /
>> +#endif
>> };
>>
>> #define INIT_THREAD \
>> diff --git a/arch/um/kernel/Makefile b/arch/um/kernel/Makefile
>> index 811188be954c..c616e884a488 100644
>> --- a/arch/um/kernel/Makefile
>> +++ b/arch/um/kernel/Makefile
>> @@ -26,9 +26,13 @@ obj-$(CONFIG_OF) += dtb.o
>> obj-$(CONFIG_EARLY_PRINTK) += early_printk.o
>> obj-$(CONFIG_STACKTRACE) += stacktrace.o
>> obj-$(CONFIG_GENERIC_PCI_IOMAP) += ioport.o
>> +obj-$(CONFIG_PREEMPT) += fpu.o
>> +obj-$(CONFIG_PREEMPT_VOLUNTARY) += fpu.o
>>
>> USER_OBJS := config.o
>>
>> +CFLAGS_fpu.o += -mxsave -mxsaveopt
>> +
>> include $(srctree)/arch/um/scripts/Makefile.rules
>>
>> targets := config.c config.tmp capflags.c
>> diff --git a/arch/um/kernel/fpu.c b/arch/um/kernel/fpu.c
>> new file mode 100644
>> index 000000000000..346c07236185
>> --- /dev/null
>> +++ b/arch/um/kernel/fpu.c
>> @@ -0,0 +1,77 @@
>> +// SPDX-License-Identifier: GPL-2.0-only
>> +/
>> + * Copyright (C) 2023 Cambridge Greys Ltd
>> + * Copyright (C) 2023 Red Hat Inc
>> + */
>> +
>> +#include <linux/cpu.h>
>>
>> +#include <linux/init.h>
>>
>> +#include <asm/fpu/api.h>
>>
>> +#include <asm/cpufeature.h>
>>
>> +
>> +/*
>> + * The critical section between kernel_fpu_begin() and kernel_fpu_end()
>> + * is non-reentrant. It is the caller's responsibility to avoid reentrance.
>> + /
>> +
>> +static DEFINE_PER_CPU(bool, in_kernel_fpu);
>> +
>> +/ UML and driver code it pulls out of the x86 tree knows about 387 features
>> + * up to and including AVX512. TILE, etc are not yet supported.
>> + */
>> +
>> +#define KNOWN_387_FEATURES 0xFF
>> +
>> +
>> +void kernel_fpu_begin(void)
>> +{
>> + preempt_disable();
>> +
>> + WARN_ON(this_cpu_read(in_kernel_fpu));
>> +
>> + this_cpu_write(in_kernel_fpu, true);
>> +
>> +#ifdef CONFIG_64BIT
>> + if (likely(cpu_has(&boot_cpu_data, X86_FEATURE_XSAVEOPT)))
>> + __builtin_ia32_xsaveopt64(&current->thread.fpu, KNOWN_387_FEATURES);
>>
>> + else {
>> + if (likely(cpu_has(&boot_cpu_data, X86_FEATURE_XSAVE)))
>> + __builtin_ia32_xsave64(&current->thread.fpu, KNOWN_387_FEATURES);
>>
>> + else
>> + __builtin_ia32_fxsave64(&current->thread.fpu);
>>
>> + }
>> +#else
>> + if (likely(cpu_has(&boot_cpu_data, X86_FEATURE_XSAVEOPT)))
>> + __builtin_ia32_xsaveopt(&current->thread.fpu, KNOWN_387_FEATURES);
>>
>> + else {
>> + if (likely(cpu_has(&boot_cpu_data, X86_FEATURE_XSAVE)))
>> + __builtin_ia32_xsave(&current->thread.fpu, KNOWN_387_FEATURES);
>>
>> + else
>> + __builtin_ia32_fxsave(&current->thread.fpu);
>>
>> + }
>> +#endif
>> +}
>> +
>> +EXPORT_SYMBOL_GPL(kernel_fpu_begin);
>> +
>> +void kernel_fpu_end(void)
>> +{
>> + WARN_ON(!this_cpu_read(in_kernel_fpu));
>> +
>> +#ifdef CONFIG_64BIT
>> + if (likely(cpu_has(&boot_cpu_data, X86_FEATURE_XSAVE)))
>> + __builtin_ia32_xrstor64(&current->thread.fpu, KNOWN_387_FEATURES);
>>
>> + else
>> + __builtin_ia32_fxrstor64(&current->thread.fpu);
>>
>> +#else
>> + if (likely(cpu_has(&boot_cpu_data, X86_FEATURE_XSAVE)))
>> + __builtin_ia32_xrstor(&current->thread.fpu, KNOWN_387_FEATURES);
>>
>> + else
>> + __builtin_ia32_fxrstor(&current->thread.fpu);
>>
>> +#endif
>> + this_cpu_write(in_kernel_fpu, false);
>> +
>> + preempt_enable();
>> +}
>> +EXPORT_SYMBOL_GPL(kernel_fpu_end);
>> +
>> diff --git a/arch/um/kernel/irq.c b/arch/um/kernel/irq.c
>> index 635d44606bfe..c02525da45df 100644
>> --- a/arch/um/kernel/irq.c
>> +++ b/arch/um/kernel/irq.c
>> @@ -195,7 +195,9 @@ static void _sigio_handler(struct uml_pt_regs *regs,
>>
>> void sigio_handler(int sig, struct siginfo *unused_si, struct uml_pt_regs *regs)
>> {
>> + preempt_disable();
>> _sigio_handler(regs, irqs_suspended);
>> + preempt_enable();
>> }
>>
>> static struct irq_entry *get_irq_entry_by_fd(int fd)
> Everything other than the typos look good to me. I still think that there
> should be some limits on reflection, at least when fxsave is being used
> (when running on a poorly-confgured VM with AVX and no xsave, for example).
>
> Cheers,
>
> Peter Lafreniere
> <peter@n8pjl.ca>
>
>
-- 
Anton R. Ivanov
Cambridgegreys Limited. Registered in England. Company Number 10273661
https://www.cambridgegreys.com/


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