Re: [Xen-devel] [PATCH 4/4] x86/vmx: Drop vmx_msr_state infrastructure

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 13.02.17 at 17:22, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 13/02/17 16:12, Andrew Cooper wrote:
>> On 13/02/17 16:01, Jan Beulich wrote:
>>>>>> On 13.02.17 at 15:32, <andrew.cooper3@xxxxxxxxxx> wrote:
>>>> To avoid leaking host MSR state into guests, guest LSTAR, STAR and
>>>> SYSCALL_MASK state is unconditionally loaded when switching into guest
>>>> context.
>>>>
>>>> Attempting to dirty-track the state is pointless; host state is always
>>>> restoring upon exit from guest context, meaning that guest state is always
>>>> considered dirty.
>>>>
>>>> Drop struct vmx_msr_state, enum VMX_INDEX_MSR_* and msr_index[].  The 
>>>> guests
>>>> MSR values are stored plainly in arch_vmx_struct, in the same way as 
>>>> shadow_gs
>>>> and cstar are.  vmx_restore_guest_msrs() and long_mode_do_msr_write() 
>>>> ensure
>>>> that the hardware MSR values are always up-to-date.
>>>>
>>>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>>> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
>>>
>>> However, the description above made me think whether always
>>> saving/restoring these MSRs is really needed (and desirable):
>>> We don't need the host values in place unless we context switch
>>> to a PV guest, so perhaps we should rather write them in
>>> paravirt_ctxt_switch_to()?
>> That would leak the values between different HVM guests.
>>
>> In principle we could skip the update if context switching to the idle
>> cpu, but that would involve leaking a VT-x-ism into the common code. 
>> SVM on the other hand automatically switches these MSRs on all
>> vmentries/exits so Xen always has its MSRs in context.
> 
> Furthermore, I did consider whether we should allow the guest to write
> to those MSRs directly, and handle them like shadow_gs.
> 
> I don't expect a plain OS to change them after initial setup, but a
> nested hypervisor (particularly Xen) is taking quite a performance hit
> on its context switch path because of these MSRs being intercepted at L0.

Ah, yes, that would likely help quite a bit.

Jan


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