Re: [RFC PATCH v6 04/26] x86/fpu/xstate: Introduce XSAVES system states

Borislav Petkov
2018-12-04T16:01:52Z

On Mon, Nov 19, 2018 at 01:47:47PM -0800, Yu-cheng Yu wrote:
> Control-flow Enforcement (CET) MSR contents are XSAVES system states.
> To support CET, introduce XSAVES system states first.
>
> 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 | 4 +-
> arch/x86/kernel/fpu/core.c | 6 +-
> arch/x86/kernel/fpu/init.c | 10 ---
> arch/x86/kernel/fpu/xstate.c | 94 +++++++++++++++++++----------
> 5 files changed, 69 insertions(+), 48 deletions(-)

...

> @@ -704,6 +710,7 @@ static int 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);
> fpu__xstate_clear_all_cpu_caps();
> @@ -717,6 +724,8 @@ void __init fpu__init_system_xstate(void)
> {
> unsigned int eax, ebx, ecx, edx;
> static int on_boot_cpu __initdata = 1;
> + u64 cpu_system_xfeatures_mask;
> + u64 cpu_user_xfeatures_mask;

So what I had in mind is to not have those local vars but use
xfeatures_mask_user and xfeatures_mask_system here directly...

> int err;
> int i;
>
> @@ -739,10 +748,23 @@ 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);
> + cpu_user_xfeatures_mask = 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);
> + cpu_system_xfeatures_mask = ecx + ((u64)edx << 32);
> +
> + xfeatures_mask_all = cpu_user_xfeatures_mask | cpu_system_xfeatures_mask;

... and not introduce xfeatures_mask_all at all but everywhere you need
all features, to do:

(xfeatures_mask_user | xfeatures_mask_system)

and work with that.

...

> @@ -1178,7 +1208,7 @@ int copy_kernel_to_xstate(struct xregs_state *xsave, const void *kbuf)
> * The state that came in from userspace was user-state only.
> * Mask all the user states out of 'xfeatures':
> */
> - xsave->header.xfeatures &= XFEATURE_MASK_SUPERVISOR;
> + xsave->header.xfeatures &= (xfeatures_mask_all & ~xfeatures_mask_user);

... and this would be

xsave->header.xfeatures &= xfeatures_mask_system;

>
> /*
> * Add back in the features that came in from userspace:
> @@ -1234,7 +1264,7 @@ int copy_user_to_xstate(struct xregs_state *xsave, const void __user *ubuf)
> * The state that came in from userspace was user-state only.
> * Mask all the user states out of 'xfeatures':
> */
> - xsave->header.xfeatures &= XFEATURE_MASK_SUPERVISOR;
> + xsave->header.xfeatures &= (xfeatures_mask_all & ~xfeatures_mask_user);

Ditto here.

This way you have *two* mask variables and code queries them only.

Hmmm?

Or am I missing something?

--
Regards/Gruss,
Boris.

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