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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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.

~Andrew

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