Re: [PATCH v2 2/3] KVM: x86: prevent setting unsupported XSAVE states

[Gleb Natapov <gleb@redhat.com>]

On Mon, Sep 23, 2013 at 12:22:37PM +0200, Paolo Bonzini wrote:
> >> @@ -6940,6 +6948,9 @@ int kvm_arch_vcpu_init(struct kvm_vcpu *vcpu)
> >>  
> >>  	vcpu->arch.ia32_tsc_adjust_msr = 0x0;
> >>  	vcpu->arch.pv_time_enabled = false;
> >> +
> >> +	vcpu->arch.supported_xcr0 = XSTATE_FPSSE;
> >> +
> > Why is this needed? It will make make __kvm_set_xcr() succeed if attempt
> > is made to set SSE bit when it is not supported in cpuid. This may not
> > be an issue in practice, but for clarity it is better for supported_xcr0
> > to contain only what is supported in guest's cpuid bits and handle the
> > fact that FP/SSE state should always be copied to/from userspace in
> > kvm_vcpu_ioctl_x86_(set|get)_xsave functions.
> 
> I don't think it makes sense to disable SSE but not XSAVE.  Linux even
> has this:
> 
>         if ((pcntxt_mask & XSTATE_FPSSE) != XSTATE_FPSSE) {
>                 pr_err("FP/SSE not shown under xsave features 0x%llx\n",
>                        pcntxt_mask);
>                 BUG();
>         }
>
That's why I said it may not be an issue in practice. What makes the
code confusing to me is that it is not clear what .supported_xcr0
contains. Why not make it contain only guest cpuid bits and use
XSTATE_FPSSE explicitly in get_xsave. I mean drop this from next patch:

        vcpu->arch.supported_xcr0 =
-               (best->eax | ((u64)best->edx << 32)) &
+               (best->eax | ((u64)best->edx << 32) | XSTATE_FPSSE) &
                host_xcr0 & KVM_SUPPORTED_XCR0;

And change
*(u64 *)&guest_xsave->region[XSAVE_HDR_OFFSET / sizeof(u32)] !=  vcpu->arch.supported_xcr0;
to
*(u64 *)&guest_xsave->region[XSAVE_HDR_OFFSET / sizeof(u32)] != (vcpu->arch.supported_xcr0 | XSTATE_FPSSE);


> I preferred this to adding dubiously-testable code that probed CPUID for
> SSE support (or should I check FXSR instead?)
> 
Not sure I understand.

--
			Gleb.