[PATCH v6 06/12] x86: add new features for paravirt patching

Juergen Gross via Virtualization
2021-03-09T13:48:31Z

For being able to switch paravirt patching from special cased custom
code sequences to ALTERNATIVE handling some X86_FEATURE_* are needed
as new features. This enables to have the standard indirect pv call
as the default code and to patch that with the non-Xen custom code
sequence via ALTERNATIVE patching later.

Make sure paravirt patching is performed before alternative patching.

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
V3:
- add comment (Boris Petkov)
- no negative features (Boris Petkov)
V4:
- move paravirt_set_cap() to paravirt-spinlocks.c
---
arch/x86/include/asm/cpufeatures.h | 2 ++
arch/x86/include/asm/paravirt.h | 10 ++++++++++
arch/x86/kernel/alternative.c | 30 ++++++++++++++++++++++++++--
arch/x86/kernel/paravirt-spinlocks.c | 9 +++++++++
4 files changed, 49 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index cc96e26d69f7..b440c950246d 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -236,6 +236,8 @@
#define X86_FEATURE_EPT_AD ( 8*32+17) /* Intel Extended Page Table access-dirty bit */
#define X86_FEATURE_VMCALL ( 8*32+18) /* "" Hypervisor supports the VMCALL instruction */
#define X86_FEATURE_VMW_VMMCALL ( 8*32+19) /* "" VMware prefers VMMCALL hypercall instruction */
+#define X86_FEATURE_PVUNLOCK ( 8*32+20) /* "" PV unlock function */
+#define X86_FEATURE_VCPUPREEMPT ( 8*32+21) /* "" PV vcpu_is_preempted function */

/* Intel-defined CPU features, CPUID level 0x00000007:0 (EBX), word 9 */
#define X86_FEATURE_FSGSBASE ( 9*32+ 0) /* RDFSBASE, WRFSBASE, RDGSBASE, WRGSBASE instructions*/
diff --git a/arch/x86/include/asm/paravirt.h b/arch/x86/include/asm/paravirt.h
index 1e45b46fae84..8c354099d9c3 100644
--- a/arch/x86/include/asm/paravirt.h
+++ b/arch/x86/include/asm/paravirt.h
@@ -47,6 +47,10 @@ static inline u64 paravirt_steal_clock(int cpu)
return static_call(pv_steal_clock)(cpu);
}

+#ifdef CONFIG_PARAVIRT_SPINLOCKS
+void __init paravirt_set_cap(void);
+#endif
+
/* The paravirtualized I/O functions */
static inline void slow_down_io(void)
{
@@ -811,5 +815,11 @@ static inline void paravirt_arch_exit_mmap(struct mm_struct *mm)
{
}
#endif
+
+#ifndef CONFIG_PARAVIRT_SPINLOCKS
+static inline void paravirt_set_cap(void)
+{
+}
+#endif
#endif /* __ASSEMBLY__ */
#endif /* _ASM_X86_PARAVIRT_H */
diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index d8e669a1546f..1f12901e75f2 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -28,6 +28,7 @@
#include <asm/insn.h>
#include <asm/io.h>
#include <asm/fixmap.h>
+#include <asm/paravirt.h>

int __read_mostly alternatives_patched;

@@ -732,6 +733,33 @@ void __init alternative_instructions(void)
* patching.
*/

+ /*
+ * Paravirt patching and alternative patching can be combined to
+ * replace a function call with a short direct code sequence (e.g.
+ * by setting a constant return value instead of doing that in an
+ * external function).
+ * In order to make this work the following sequence is required:
+ * 1. set (artificial) features depending on used paravirt
+ * functions which can later influence alternative patching
+ * 2. apply paravirt patching (generally replacing an indirect
+ * function call with a direct one)
+ * 3. apply alternative patching (e.g. replacing a direct function
+ * call with a custom code sequence)
+ * Doing paravirt patching after alternative patching would clobber
+ * the optimization of the custom code with a function call again.
+ */
+ paravirt_set_cap();
+
+ /*
+ * First patch paravirt functions, such that we overwrite the indirect
+ * call with the direct call.
+ */
+ apply_paravirt(__parainstructions, __parainstructions_end);
+
+ /*
+ * Then patch alternatives, such that those paravirt calls that are in
+ * alternatives can be overwritten by their immediate fragments.
+ */
apply_alternatives(__alt_instructions, __alt_instructions_end);

#ifdef CONFIG_SMP
@@ -750,8 +778,6 @@ void __init alternative_instructions(void)
}
#endif

- apply_paravirt(__parainstructions, __parainstructions_end);
-
restart_nmi();
alternatives_patched = 1;
}
diff --git a/arch/x86/kernel/paravirt-spinlocks.c b/arch/x86/kernel/paravirt-spinlocks.c
index 4f75d0cf6305..9e1ea99ad9df 100644
--- a/arch/x86/kernel/paravirt-spinlocks.c
+++ b/arch/x86/kernel/paravirt-spinlocks.c
@@ -32,3 +32,12 @@ bool pv_is_native_vcpu_is_preempted(void)
return pv_ops.lock.vcpu_is_preempted.func ==
__raw_callee_save___native_vcpu_is_preempted;
}
+
+void __init paravirt_set_cap(void)
+{
+ if (!pv_is_native_spin_unlock())
+ setup_force_cpu_cap(X86_FEATURE_PVUNLOCK);
+
+ if (!pv_is_native_vcpu_is_preempted())
+ setup_force_cpu_cap(X86_FEATURE_VCPUPREEMPT);
+}