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SubjectRe: [PATCH v11 7/9] KVM: X86: Add userspace access interface for CET MSRs
On Sat, Apr 25, 2020 at 05:31:59PM +0200, Paolo Bonzini wrote:
> On 26/03/20 09:18, Yang Weijiang wrote:
> > There're two different places storing Guest CET states, states
> > managed with XSAVES/XRSTORS, as restored/saved
> > in previous patch, can be read/write directly from/to the MSRs.
> > For those stored in VMCS fields, they're access via vmcs_read/
> > vmcs_write.
> >
> > To correctly read/write the CET MSRs, it's necessary to check
> > whether the kernel FPU context switch happened and reload guest
> > FPU context if needed.
>
> I have one question here, it may be just a misunderstanding.
>
> As I understand it, the PLx_SSP MSRs are only used when the current
> privilege level changes; the processor has a hidden SSP register for the
> current privilege level, and the SSP can be accessed via VMCS only.
>
> These patches do not allow saving/restoring this hidden register.
> However, this should be necessary in order to migrate the virtual
> machine. The simplest way to plumb this is through a KVM-specific MSR
> in arch/x86/include/uapi/asm/kvm_para.h. This MSR should only be
> accessible to userspace, i.e. only if msr_info->host_initiated.
Thanks for raising the issue!
I checked SDM again, yes, it's neccessary to save the current SSP for
migration case, I'll follow your advice to add it as custom MSR.

> Testing CET in the state-test selftest is a bit hard because you have to
> set up S_CET and the shadow stack, but it would be great to have a
> separate test similar to tools/testing/selftests/x86_64/smm_test. It's
> not an absolute requirement for merging, but if you can put it on your
> todo list it would be better.
>
What's the purpose of the selftest? Is it just for Shadow Stack SSP
state transitions in various cases? e.g., L0 SSP<--->L3 SSP,
L0 SSP1<--->L0 SSP2? We now have the KVM unit-test for CET functionalities,
i.e., Shadow Stack and Indirect Branch Tracking for user-mode, I can put the
state test app into the todo list as current patchset is mainly for user-mode
protection, the supervisor-mode CET protection is the next step.

> Thanks,
>
> Paolo

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