Re: [PATCH v8 04/27] x86/fpu/xstate: Introduce XSAVES system states

Thomas Gleixner
2019-08-16T19:56:40Z

On Tue, 13 Aug 2019, Yu-cheng Yu wrote:
> +/*
> + * On context switches, XSAVE states are not restored until returning
> + * to user-mode. FPU registers need to be restored before any changes,
> + * and protected by fpregs_lock()/fpregs_unlock().

I really had to read this comment twice to figure out what it means.

> + */
> +static inline void modify_fpu_regs_begin(void)

Please use a proper name space. fpu_regs_....

> +{
> + fpregs_lock();
> + if (test_thread_flag(TIF_NEED_FPU_LOAD))
> + __fpregs_load_activate();
> +}
> +
> +static inline void modify_fpu_regs_end(void)
> +{
> + fpregs_unlock();
> +}

Also why are those inlines in this particular patch? I see no relation at all.

> /*
> * MXCSR and XCR definitions:
> */
> diff --git a/arch/x86/include/asm/fpu/xstate.h b/arch/x86/include/asm/fpu/xstate.h
> index 9ded9532257d..970bbd303cfb 100644
> --- a/arch/x86/include/asm/fpu/xstate.h
> +++ b/arch/x86/include/asm/fpu/xstate.h
> @@ -21,9 +21,6 @@
> #define XSAVE_YMM_SIZE 256
> #define XSAVE_YMM_OFFSET (XSAVE_HDR_SIZE + XSAVE_HDR_OFFSET)
>
> -/* Supervisor features */
> -#define XFEATURE_MASK_SUPERVISOR (XFEATURE_MASK_PT)
> -
> /* All currently supported features */
> #define SUPPORTED_XFEATURES_MASK (XFEATURE_MASK_FP | \
> XFEATURE_MASK_SSE | \
> @@ -42,6 +39,7 @@
> #endif
>
> extern u64 xfeatures_mask_user;
> +extern u64 xfeatures_mask_all;
> extern u64 xstate_fx_sw_bytes[USER_XSTATE_FX_SW_WORDS];
>
> extern void __init update_regset_xstate_info(unsigned int size,
> diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
> index 12c70840980e..31d3cd70b5df 100644
> --- a/arch/x86/kernel/fpu/core.c
> +++ b/arch/x86/kernel/fpu/core.c
> @@ -294,12 +294,16 @@ void fpu__drop(struct fpu *fpu)
> * Clear FPU registers by setting them up from
> * the init fpstate:
> */
> -static inline void copy_init_fpstate_to_fpregs(void)
> +static inline void copy_init_fpstate_to_fpregs(u64 features_mask)
> {
> fpregs_lock();
>
> + /*
> + * Only XSAVES user states are copied.
> + * System states are preserved.

Fits nicely in one line and aside of that this comment is blatantly
wrong. See that caller:

> + copy_init_fpstate_to_fpregs(xfeatures_mask_all);

xfeatures_mask_all includes xfeatures_mask_system unless I'm missing
something.

> + */
> if (use_xsave())
> - copy_kernel_to_xregs(&init_fpstate.xsave, -1);
> + copy_kernel_to_xregs(&init_fpstate.xsave, features_mask);
> else if (static_cpu_has(X86_FEATURE_FXSR))
> copy_kernel_to_fxregs(&init_fpstate.fxsave);

The change of this function should also be split out into a separate
patch. This one is way too big to be reviewable.

> else
> @@ -318,7 +322,21 @@ static inline void copy_init_fpstate_to_fpregs(void)
> * Called by sys_execve(), by the signal handler code and by various
> * error paths.
> */
> -void fpu__clear(struct fpu *fpu)
> +void fpu__clear_user_states(struct fpu *fpu)
> +{
> + WARN_ON_FPU(fpu != &current->thread.fpu); /* Almost certainly an anomaly */

1) Please do not use tail comments. They break the reading flow.

2) Please do not comment the obvious. Put comments where they make sense. I
know you copied it, but that does not make it any better.

> + fpu__drop(fpu);
> +
> + /*
> + * Make sure fpstate is cleared and initialized.
> + */
> + fpu__initialize(fpu);
> + if (static_cpu_has(X86_FEATURE_FPU))
> + copy_init_fpstate_to_fpregs(xfeatures_mask_user);
> +}
> +
> +void fpu__clear_all(struct fpu *fpu)
> {
> WARN_ON_FPU(fpu != &current->thread.fpu); /* Almost certainly an anomaly */
> @@ -329,7 +347,7 @@ void fpu__clear(struct fpu *fpu)
> */
> fpu__initialize(fpu);
> if (static_cpu_has(X86_FEATURE_FPU))
> - copy_init_fpstate_to_fpregs();
> + copy_init_fpstate_to_fpregs(xfeatures_mask_all);
> }
>
> /*
> diff --git a/arch/x86/kernel/fpu/init.c b/arch/x86/kernel/fpu/init.c
> index 73fed33e5bda..0a0ba584a533 100644
> --- a/arch/x86/kernel/fpu/init.c
> +++ b/arch/x86/kernel/fpu/init.c
> @@ -217,16 +217,6 @@ static void __init fpu__init_system_xstate_size_legacy(void)
> fpu_user_xstate_size = fpu_kernel_xstate_size;
> }
>
> -/*
> - * Find supported xfeatures based on cpu features and command-line input.
> - * This must be called after fpu__init_parse_early_param() is called and
> - * xfeatures_mask is enumerated.
> - */
> -u64 __init fpu__get_supported_xfeatures_mask(void)
> -{
> - return SUPPORTED_XFEATURES_MASK;
> -}
> -
> /* Legacy code to initialize eager fpu mode. */
> static void __init fpu__init_system_ctx_switch(void)
> {
> diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
> index 8a63f07cf400..4ecf1764a971 100644
> --- a/arch/x86/kernel/fpu/signal.c
> +++ b/arch/x86/kernel/fpu/signal.c
> @@ -285,7 +285,7 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
> IS_ENABLED(CONFIG_IA32_EMULATION));
>
> if (!buf) {
> - fpu__clear(fpu);
> + fpu__clear_user_states(fpu);
> return 0;
> }
>
> @@ -407,7 +407,7 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
>
> err_out:
> if (ret)
> - fpu__clear(fpu);
> + fpu__clear_user_states(fpu);
> return ret;
> }
>
> diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
> index d560e8861a3c..9fbe73c546df 100644
> --- a/arch/x86/kernel/fpu/xstate.c
> +++ b/arch/x86/kernel/fpu/xstate.c
> @@ -61,9 +61,19 @@ static short xsave_cpuid_features[] __initdata = {
> */
> u64 xfeatures_mask_user __read_mostly;
>
> +/*
> + * Supported XSAVES system states.
> + */
> +static u64 xfeatures_mask_system __read_mostly;
> +
> +/*
> + * Combined XSAVES system and user states.
> + */
> +u64 xfeatures_mask_all __read_mostly;
> +
> static unsigned int xstate_offsets[XFEATURE_MAX] = { [ 0 ... XFEATURE_MAX - 1] = -1};
> static unsigned int xstate_sizes[XFEATURE_MAX] = { [ 0 ... XFEATURE_MAX - 1] = -1};
> -static unsigned int xstate_comp_offsets[sizeof(xfeatures_mask_user)*8];
> +static unsigned int xstate_comp_offsets[sizeof(xfeatures_mask_all)*8];

[sizeof(...) * 8]

>
> /*
> * The XSAVE area of kernel can be in standard or compacted format;
> @@ -79,7 +89,7 @@ unsigned int fpu_user_xstate_size;
> */
> int cpu_has_xfeatures(u64 xfeatures_needed, const char **feature_name)
> {
> - u64 xfeatures_missing = xfeatures_needed & ~xfeatures_mask_user;
> + u64 xfeatures_missing = xfeatures_needed & ~xfeatures_mask_all;
>
> if (unlikely(feature_name)) {
> long xfeature_idx, max_idx;
> @@ -158,7 +168,7 @@ void fpstate_sanitize_xstate(struct fpu *fpu)
> * None of the feature bits are in init state. So nothing else
> * to do for us, as the memory layout is up to date.
> */
> - if ((xfeatures & xfeatures_mask_user) == xfeatures_mask_user)
> + if ((xfeatures & xfeatures_mask_all) == xfeatures_mask_all)
> return;
>
> /*
> @@ -213,28 +223,27 @@ void fpstate_sanitize_xstate(struct fpu *fpu)
> */
> void fpu__init_cpu_xstate(void)
> {
> - if (!boot_cpu_has(X86_FEATURE_XSAVE) || !xfeatures_mask_user)
> + if (!boot_cpu_has(X86_FEATURE_XSAVE) || !xfeatures_mask_all)
> return;
> /*
> * XCR_XFEATURE_ENABLED_MASK sets the features that are managed
> * by XSAVE{C, OPT} and XRSTOR. Only XSAVE user states can be
> * set here.
> */
> -
> - xfeatures_mask_user &= ~XFEATURE_MASK_SUPERVISOR;
> -
> cr4_set_bits(X86_CR4_OSXSAVE);
> xsetbv(XCR_XFEATURE_ENABLED_MASK, xfeatures_mask_user);
> +
> + /*
> + * MSR_IA32_XSS controls which system (not user) states are

We know that system state is not including user state. Please stop
documenting the obvious.

> + * to be managed by XSAVES.
> + */
> + if (boot_cpu_has(X86_FEATURE_XSAVES))
> + wrmsrl(MSR_IA32_XSS, xfeatures_mask_system);
> }
>
> -/*
> - * Note that in the future we will likely need a pair of
> - * functions here: one for user xstates and the other for
> - * system xstates. For now, they are the same.
> - */
> static int xfeature_enabled(enum xfeature xfeature)
> {
> - return !!(xfeatures_mask_user & BIT_ULL(xfeature));
> + return !!(xfeatures_mask_all & BIT_ULL(xfeature));
> }
>
> /*
> @@ -340,7 +349,7 @@ static int xfeature_is_aligned(int xfeature_nr)
> */
> static void __init setup_xstate_comp(void)
> {
> - unsigned int xstate_comp_sizes[sizeof(xfeatures_mask_user)*8];
> + unsigned int xstate_comp_sizes[sizeof(xfeatures_mask_all)*8];

See above

> int i;
>
> /*
> @@ -413,7 +422,7 @@ static void __init setup_init_fpu_buf(void)
> print_xstate_features();
>
> if (boot_cpu_has(X86_FEATURE_XSAVES))
> - init_fpstate.xsave.header.xcomp_bv = BIT_ULL(63) | xfeatures_mask_user;
> + init_fpstate.xsave.header.xcomp_bv = BIT_ULL(63) | xfeatures_mask_all;
>
> /*
> * Init all the features state with header.xfeatures being 0x0
> @@ -436,7 +445,7 @@ static int xfeature_uncompacted_offset(int xfeature_nr)
> * format. Checking a system state's uncompacted offset is
> * an error.
> */
> - if (XFEATURE_MASK_SUPERVISOR & BIT_ULL(xfeature_nr)) {
> + if (~xfeatures_mask_user & BIT_ULL(xfeature_nr)) {

Sigh. Why can't this use xfeatures_mask_system? That would be too obvious.

> WARN_ONCE(1, "No fixed offset for xstate %d\n", xfeature_nr);
> return -1;
> }
> @@ -608,15 +617,12 @@ static void do_extra_xstate_size_checks(void)
>
>
> /*
> - * Get total size of enabled xstates in XCR0/xfeatures_mask_user.
> + * Get total size of enabled xstates in XCR0 | IA32_XSS.
> *
> * Note the SDM's wording here. "sub-function 0" only enumerates
> * the size of the *user* states. If we use it to size a buffer
> * that we use 'XSAVES' on, we could potentially overflow the
> * buffer because 'XSAVES' saves system states too.
> - *
> - * Note that we do not currently set any bits on IA32_XSS so
> - * 'XCR0 | IA32_XSS == XCR0' for now.
> */
> static unsigned int __init get_xsaves_size(void)
> {
> @@ -698,6 +704,7 @@ static int __init init_xstate_size(void)
> */
> static void fpu__init_disable_system_xstate(void)
> {
> + xfeatures_mask_all = 0;
> xfeatures_mask_user = 0;
> cr4_clear_bits(X86_CR4_OSXSAVE);
> setup_clear_cpu_cap(X86_FEATURE_XSAVE);
> @@ -733,10 +740,23 @@ void __init fpu__init_system_xstate(void)
> return;
> }
>
> + /*
> + * Find user states supported by the processor.
> + * Only these bits can be set in XCR0.
> + */
> cpuid_count(XSTATE_CPUID, 0, &eax, &ebx, &ecx, &edx);
> xfeatures_mask_user = eax + ((u64)edx << 32);
>
> - if ((xfeatures_mask_user & XFEATURE_MASK_FPSSE) != XFEATURE_MASK_FPSSE) {
> + /*
> + * Find system states supported by the processor.
> + * Only these bits can be set in IA32_XSS MSR.
> + */
> + cpuid_count(XSTATE_CPUID, 1, &eax, &ebx, &ecx, &edx);
> + xfeatures_mask_system = ecx + ((u64)edx << 32);
> +
> + xfeatures_mask_all = xfeatures_mask_user | xfeatures_mask_system;
> +
> + if ((xfeatures_mask_all & XFEATURE_MASK_FPSSE) != XFEATURE_MASK_FPSSE) {

xfeatures_mask_all is wrong here. FPSSE is clearly user state.

> /*
> * This indicates that something really unexpected happened
> * with the enumeration. Disable XSAVE and try to continue
> @@ -751,10 +771,12 @@ void __init fpu__init_system_xstate(void)
> */
> for (i = 0; i < ARRAY_SIZE(xsave_cpuid_features); i++) {
> if (!boot_cpu_has(xsave_cpuid_features[i]))
> - xfeatures_mask_user &= ~BIT_ULL(i);
> + xfeatures_mask_all &= ~BIT_ULL(i);
> }
>
> - xfeatures_mask_user &= fpu__get_supported_xfeatures_mask();
> + xfeatures_mask_all &= SUPPORTED_XFEATURES_MASK;
> + xfeatures_mask_user &= xfeatures_mask_all;
> + xfeatures_mask_system &= xfeatures_mask_all;
>
> /* Enable xstate instructions to be able to continue with initialization: */
> fpu__init_cpu_xstate();
> @@ -766,7 +788,7 @@ void __init fpu__init_system_xstate(void)
> * Update info used for ptrace frames; use standard-format size and no
> * system xstates:
> */
> - update_regset_xstate_info(fpu_user_xstate_size, xfeatures_mask_user & ~XFEATURE_MASK_SUPERVISOR);
> + update_regset_xstate_info(fpu_user_xstate_size, xfeatures_mask_user);
>

And exactly this hunk shows that the whole refactoring approach is wrong
from the very beginning. I stared at that in the previous patch already and
had the feeling that it's bogus.

Just doing a s/xfeatures_mask/xfeatures_mask_user/g really does not make
any sense. Simply because the current code assumes that xfeatures_mask ==
xfeatures_mask_all. So if a global rename is the right approach then
s/xfeatures_mask/xfeatures_mask_all/ and not that completely backwards
rename to _user.

That refactoring wants to be done in the following steps:

1) Introduce xfeatures_mask_user and initialize it with

xfeatures_mask_user = xfeatures_mask ^ ~XFEATURE_MASK_SUPERVISOR;

2) Fix up the usage sites in reviewable chunks. It does not matter
whether that could be folded into a larger all in one patch. What
matters is that it makes sense and is reviewable.

3) Change the signature of copy_init_fpstate_to_fpregs() so it takes a
mask and fix up the call sites accordingly. Without the bogus comment
of course.

4) Introduce xfeatures_mask_system and eventually needed helper functions.

5) Change the affected usage sites

Details may be slightly different but you get the idea.

Thanks,

tglx