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

Yu-cheng Yu
2018-12-04T17:13:44Z

On Tue, 2018-12-04 at 17:01 +0100, Borislav Petkov wrote:
> 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...

Ok, I will re-write it.

...

> >
> > @@ -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.

Then we will do this very often. Why don't we create all three in the
beginning: xfeatures_mask_all, xfeatures_mask_user, and xfeatures_mask_system?

> ...
>
> > @@ -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;

Yes.

>
> >
> > /*
> > * 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?

We actually have three.

Yu-cheng