Re: [PATCH 1/1] KVM: X86: add the support of XSAVE/XRSTOR to guest

[Avi Kivity <avi@redhat.com>]

On 05/06/2010 05:20 PM, Cui, Dexuan wrote:
>
>> However, switching xcr0 may be slow.  That's our experience with msrs.
>> Can you measure its latency?
>>      
> We can measure that.
> However, I think the changing xcr0 to guest xcr0 in handle_xsetbv() is necessary --
> or else, inside guest xgetbv() would return host xcr0 rather than guest xcr0 --
> this is obviously incorrect. Once handle_xsetbv() changes the xcr0 to guest's value,
> the xsetbv() in kvm_fx_restore_host() is also necessary, and the xsetbv() in
> kvm_fx_restore_guest() is also necessary. So looks guest can't run with the
> host xcr0.
>    

Right.  Moreover, xsave would write into memory the guest doesn't expect.

>> btw, it needs save/restore for live migration, as well as save/restore
>> for the new fpu state.
>>      
> Yes. This part is missing. Sheng and I is also doing this -- it may be a bittle
> troublesome as the number of XSTATEs can grown as time goes on. We'll
> have to handle the compatibility issue.
>    


Reserve tons of space in the ioctl - and we can use the same format as 
xsave.

All those control registers are annoying, we have cr1 and cr5-cr7 free, 
cr9-cr15 on x86_64, infinite msr space, and now we have XCRs.  Great.

Looking forward to YCR0.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.