Re: [RFC PATCH v4 03/27] x86/fpu/xstate: Enable XSAVES system states

Borislav Petkov
2018-10-02T17:15:55Z

On Fri, Sep 21, 2018 at 08:03:27AM -0700, Yu-cheng Yu wrote:
> XSAVES saves both system and user states. The Linux kernel
> currently does not save/restore any system states. This patch
> creates the framework for supporting system states.

... and needs a lot more text explaining *why* it is doing that.

>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
> ---
> arch/x86/include/asm/fpu/internal.h | 3 +-
> arch/x86/include/asm/fpu/xstate.h | 9 ++-
> arch/x86/kernel/fpu/core.c | 7 +-
> arch/x86/kernel/fpu/init.c | 10 ---
> arch/x86/kernel/fpu/xstate.c | 112 +++++++++++++++++-----------
> 5 files changed, 80 insertions(+), 61 deletions(-)
>
> diff --git a/arch/x86/include/asm/fpu/internal.h b/arch/x86/include/asm/fpu/internal.h
> index f1f9bf91a0ab..1f447865db3a 100644
> --- a/arch/x86/include/asm/fpu/internal.h
> +++ b/arch/x86/include/asm/fpu/internal.h
> @@ -45,7 +45,6 @@ extern void fpu__init_cpu_xstate(void);
> extern void fpu__init_system(struct cpuinfo_x86 *c);
> extern void fpu__init_check_bugs(void);
> extern void fpu__resume_cpu(void);
> -extern u64 fpu__get_supported_xfeatures_mask(void);
>
> /*
> * Debugging facility:
> @@ -94,7 +93,7 @@ static inline void fpstate_init_xstate(struct xregs_state *xsave)
> * trigger #GP:
> */
> xsave->header.xcomp_bv = XCOMP_BV_COMPACTED_FORMAT |
> - xfeatures_mask_user;
> + xfeatures_mask_all;
> }
>
> static inline void fpstate_init_fxstate(struct fxregs_state *fx)
> diff --git a/arch/x86/include/asm/fpu/xstate.h b/arch/x86/include/asm/fpu/xstate.h
> index 9b382e5157ed..a32dc5f8c963 100644
> --- a/arch/x86/include/asm/fpu/xstate.h
> +++ b/arch/x86/include/asm/fpu/xstate.h
> @@ -19,10 +19,10 @@
> #define XSAVE_YMM_SIZE 256
> #define XSAVE_YMM_OFFSET (XSAVE_HDR_SIZE + XSAVE_HDR_OFFSET)
>
> -/* System features */
> -#define XFEATURE_MASK_SYSTEM (XFEATURE_MASK_PT)

Previous patch renames it, this patch deletes it. Why do we need all
that unnecessary churn?

Also, this patch is trying to do a couple of things at once and
reviewing it is not trivial. Please split the changes logically.

> diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
> index 19f8df54c72a..dd2c561c4544 100644
> --- a/arch/x86/kernel/fpu/xstate.c
> +++ b/arch/x86/kernel/fpu/xstate.c
> @@ -51,13 +51,16 @@ static short xsave_cpuid_features[] __initdata = {
> };
>
> /*
> - * Mask of xstate features supported by the CPU and the kernel:
> + * Mask of xstate features supported by the CPU and the kernel.
> + * This is the result from CPUID query, SUPPORTED_XFEATURES_MASK,
> + * and boot_cpu_has().
> */

This needs to explain what both masks are - user and system. "CPU" and
"kernel" is not "user" and "all".

> u64 xfeatures_mask_user __read_mostly;
> +u64 xfeatures_mask_all __read_mostly;

> @@ -219,30 +222,31 @@ 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;
> +
> + cr4_set_bits(X86_CR4_OSXSAVE);
> +
> /*
> - * Make it clear that XSAVES system states are not yet
> - * implemented should anyone expect it to work by changing
> - * bits in XFEATURE_MASK_* macros and XCR0.
> + * XCR_XFEATURE_ENABLED_MASK sets the features that are managed
> + * by XSAVE{C, OPT} and XRSTOR. Only XSAVE user states can be
> + * set here.
> */
> - WARN_ONCE((xfeatures_mask_user & XFEATURE_MASK_SYSTEM),
> - "x86/fpu: XSAVES system states are not yet implemented.\n");
> + xsetbv(XCR_XFEATURE_ENABLED_MASK,
> + xfeatures_mask_user);

No need to break the line here.

Also, you have a couple more places in your patches where you
unnecessarily break lines. Please don't do that, even if it exceeds 80
cols by a couple of chars.

>
> - xfeatures_mask_user &= ~XFEATURE_MASK_SYSTEM;
> -
> - cr4_set_bits(X86_CR4_OSXSAVE);
> - xsetbv(XCR_XFEATURE_ENABLED_MASK, xfeatures_mask_user);
> + /*
> + * MSR_IA32_XSS sets which XSAVES system states to be managed by
> + * XSAVES. Only XSAVES system states can be set here.
> + */
> + if (boot_cpu_has(X86_FEATURE_XSAVES))
> + wrmsrl(MSR_IA32_XSS,
> + xfeatures_mask_all & ~xfeatures_mask_user);

--
Regards/Gruss,
Boris.

Good mailing practices for 400: avoid top-posting and trim the reply.