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

[Wanpeng Li <wanpeng.li@linux.intel.com>]

Hi Paolo,
On Thu, Aug 21, 2014 at 01:56:46PM +0200, Paolo Bonzini wrote:
>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

I fail to understand "they are passed as zero". Could you explain more
in details? ;-)

Regards,
Wanpeng Li 

>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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