Re: [PATCH] KVM: x86: Keep masked bits unmodified on kvm_set_shared_msr

[Paolo Bonzini <pbonzini@redhat.com>]

Il 21/08/2014 10:05, Wanpeng Li ha scritto:
> Hi Nadav,
> On Wed, Aug 20, 2014 at 03:11:51PM +0300, Nadav Amit wrote:
>> Currently, when an msr is updated using kvm_set_shared_msr the masked bits are
>> zeroed.  This behavior is currently valid since the only MSR with partial mask
> 
> Why zeroed? vmx_vcpu_setup() set all mask to -1ull.

He meant they are passed as zero in the WRMSR but actually they're not
zeroed.  They're set to the value that is passed to kvm_set_shared_msr,
and this value is massaged elsewhere to do mix guest and host bugs.  See
update_transition_efer.

So I'm removing this patch, it's wrong.

Paolo

>> is EFER, in which only SCE might be unmasked. However, using the
> 
> Do you mean SCE might be masked? 
> 
>> kvm_set_shared_msr for other purposes becomes impossible.
>>
>> This patch keeps the masked bits unmodified while setting a shared msr.
>>
> 
> Do you mean "keeps the unmasked bits unmodified" instead of "keeps the
> masked bits unmodified"?
> 
> Regards,
> Wanpeng Li 
> 
>> Signed-off-by: Nadav Amit <namit@cs.technion.ac.il>
>> ---
>> arch/x86/kvm/x86.c | 1 +
>> 1 file changed, 1 insertion(+)
>>
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index 5f5edb6..ee42410 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -236,6 +236,7 @@ void kvm_set_shared_msr(unsigned slot, u64 value, u64 mask)
>>
>> 	if (((value ^ smsr->values[slot].curr) & mask) == 0)
>> 		return;
>> +	value = (smsr->values[slot].curr & ~mask) | (value & mask);
>> 	smsr->values[slot].curr = value;
>> 	wrmsrl(shared_msrs_global.msrs[slot], value);
>> 	if (!smsr->registered) {
>> -- 
>> 1.9.1
>>
>> --
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