RE: [patch 5/6] x86/fpu: Provide fpu_update_guest_xcr0/xfd()

[Thomas Gleixner]

On Thu, Dec 16 2021 at 09:59, Kevin Tian wrote:
>> From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>> This can be done simply with the MSR entry/exit controls. No trap
>> required neither for #NM for for XFD_ERR.
>> 
>> VMENTER loads guest state. VMEXIT saves guest state and loads host state
>> (0)
>
> This implies three MSR operations for every vm-exit.
>
> With trap we only need one RDMSR in host #NM handler, one 
> RDMSR/one WRMSR exit in guest #NM handler, which are both rare.
> plus one RDMSR/one WRMSR per vm-exit only if saved xfd_err is 
> non-zero which is again rare.

Fair enough.

>> XFD:     Always guest state
>> 
>> So VMENTER does nothing and VMEXIT either saves guest state and the sync
>> function uses the automatically saved value or you keep the sync
>> function which does the rdmsrl() as is.
>> 
>
> Yes, this is the 3rd open that I asked in another reply. The only restriction
> with this approach is that the sync cost is added also for legacy OS which
> doesn't touch xfd at all. 

You still can make that conditional on the guest XCR0. If guest never
enables the extended bit then neither the #NM trap nor the XFD sync
are required.

But yes, there are too many moving parts here :)

Thanks,

        tglx