From: Gleb Natapov <gleb@redhat.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 2/3] KVM: x86: prevent setting unsupported XSAVE states
Date: Mon, 23 Sep 2013 18:09:55 +0300 [thread overview]
Message-ID: <20130923150955.GB10353@redhat.com> (raw)
In-Reply-To: <5240166D.5020401@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.
next prev parent reply other threads:[~2013-09-23 15:09 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-09-16 14:26 [PATCH v2 0/3] KVM: prepare for future XSAVE extensions Paolo Bonzini
2013-09-16 14:26 ` [PATCH v2 1/3] KVM: x86: mask unsupported XSAVE entries from leaf 0Dh index 0 Paolo Bonzini
2013-09-16 14:26 ` [PATCH v2 2/3] KVM: x86: prevent setting unsupported XSAVE states Paolo Bonzini
2013-09-22 9:33 ` Gleb Natapov
2013-09-23 10:22 ` Paolo Bonzini
2013-09-23 15:09 ` Gleb Natapov [this message]
2013-09-23 15:24 ` Paolo Bonzini
2013-09-16 14:26 ` [PATCH v2 3/3] KVM: x86: only copy XSAVE state for the supported features Paolo Bonzini
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