On 11/30/2020 9:45 AM, Dave Hansen wrote:
On 11/10/20 8:21 AM, Yu-cheng Yu wrote:
Control-flow Enforcement Technology (CET) adds five MSRs. Introduce
them and their XSAVES supervisor states:
MSR_IA32_U_CET (user-mode CET settings),
MSR_IA32_PL3_SSP (user-mode Shadow Stack pointer),
MSR_IA32_PL0_SSP (kernel-mode Shadow Stack pointer),
MSR_IA32_PL1_SSP (Privilege Level 1 Shadow Stack pointer),
MSR_IA32_PL2_SSP (Privilege Level 2 Shadow Stack pointer).
This patch goes into a bunch of XSAVE work that this changelog only
briefly touches on. I think it needs to be beefed up a bit.
I will do that.
@@ -835,8 +843,19 @@ void __init fpu__init_system_xstate(void)
* Clear XSAVE features that are disabled in the normal CPUID.
*/
for (i = 0; i < ARRAY_SIZE(xsave_cpuid_features); i++) {
- if (!boot_cpu_has(xsave_cpuid_features[i]))
- xfeatures_mask_all &= ~BIT_ULL(i);
+ if (xsave_cpuid_features[i] == X86_FEATURE_SHSTK) {
+ /*
+ * X86_FEATURE_SHSTK and X86_FEATURE_IBT share
+ * same states, but can be enabled separately.
+ */
+ if (!boot_cpu_has(X86_FEATURE_SHSTK) &&
+ !boot_cpu_has(X86_FEATURE_IBT))
+ xfeatures_mask_all &= ~BIT_ULL(i);
+ } else {
+ if ((xsave_cpuid_features[i] == -1) ||
Where did the -1 come from? Was that introduced earlier in this series?
I don't see any way a xsave_cpuid_features[] can be -1 in the current tree.
Yes, we used to have a hole in xsave_cpuid_features[] and put -1 there.
Do we want to keep this in case we again have holes in the future?
+ !boot_cpu_has(xsave_cpuid_features[i]))
+ xfeatures_mask_all &= ~BIT_ULL(i);
+ }
}
Do we have any other spots in the kernel where we care about:
boot_cpu_has(X86_FEATURE_SHSTK) ||
boot_cpu_has(X86_FEATURE_IBT)
? If so, we could also address this by declaring a software-defined
X86_FEATURE_CET and then setting it if SHSTK||IBT is supported, then we
just put that one feature in xsave_cpuid_features[].
That is a better solution. I will look into that.
I'm also not crazy about the loop as it is. I'd much rather see this in
a helper like:
bool cpu_supports_xsave_deps(int xfeature)
{
bool ret;
ret = boot_cpu_has(xsave_cpuid_features[xfeature])
/*
* X86_FEATURE_SHSTK is checked in xsave_cpuid_features()
* but the CET states are needed if either SHSTK or IBT are
* available.
*/
if (xfeature == XFEATURE_CET_USER ||
xfeature == XFEATURE_CET_KERNEL)
ret |= boot_cpu_has(X86_FEATURE_IBT)
return ret;
}
See how that's extensible? You can add as many special cases as you want.
Yes.
Thanks,
Yu-cheng
