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SubjectRe: [PATCH 2/2] KVM: x86: fix KVM_SET_XCRS loop
Il 18/10/2013 02:04, Marcelo Tosatti ha scritto:
> On Thu, Oct 17, 2013 at 04:50:47PM +0200, Paolo Bonzini wrote:
>> The loop was always using 0 as the index. This means that
>> any rubbish after the first element of the array went undetected.
>> It seems reasonable to assume that no KVM userspace did that.
>
> It is not a typo, look at __kvm_set_xcr when setting
> guest_xcrs->xcrs[i].value, with i != 0.

i is not the index of the XCR register, it's the index in the array.

The index is currently hardcoded to 0 when it is passed to
__kvm_set_xcr; but very reasonably __kvm_set_xcr returns 1 when index != 0.

IMO even the "if" in kvm_vcpu_ioctl_x86_set_xcrs is wrong: setting XCR
above XCR0 should fail KVM_SET_XCRS, while currently is ignored. The
body of the loop should be simply:

r = __kvm_set_xcr(vcpu, guest_xcrs->xcrs[i].xcr,
guest_xcrs->xcrs[i].value);
if (r)
break;

Paolo

> The code is not prepared to deal with XCR != 0 (because its not
> implemented in hw).
>
>> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
>> ---
>> arch/x86/kvm/x86.c | 4 ++--
>> 1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index f4e1391..f91dff2 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -3062,9 +3062,9 @@ static int kvm_vcpu_ioctl_x86_set_xcrs(struct kvm_vcpu *vcpu,
>>
>> for (i = 0; i < guest_xcrs->nr_xcrs; i++)
>> /* Only support XCR0 currently */
>> - if (guest_xcrs->xcrs[0].xcr == XCR_XFEATURE_ENABLED_MASK) {
>> + if (guest_xcrs->xcrs[i].xcr == XCR_XFEATURE_ENABLED_MASK) {
>> r = __kvm_set_xcr(vcpu, XCR_XFEATURE_ENABLED_MASK,
>> - guest_xcrs->xcrs[0].value);
>> + guest_xcrs->xcrs[i].value);
>> break;
>> }
>> if (r)
>> --
>> 1.8.3.1



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