Re: [PATCH] x86/intel: Expose MSR_ARCH_CAPS to dom0

[Andrew Cooper <amc96@xxxxxxxxx>]

On 28/08/2020 17:17, Jan Beulich wrote:
> On 28.08.2020 18:09, Andrew Cooper wrote:
>> On 28/08/2020 16:42, Jan Beulich wrote:
>>> On 28.08.2020 12:23, Andrew Cooper wrote:
>>>> On 28/08/2020 09:41, Jan Beulich wrote:
>>>>> On 27.08.2020 21:37, Andrew Cooper wrote:
>>>>>> The overhead of (the lack of) MDS_NO alone has been measured at 30% on 
>>>>>> some
>>>>>> workloads.  While we're not in a position yet to offer MSR_ARCH_CAPS 
>>>>>> generally
>>>>>> to guests, dom0 doesn't migrate, so we can pass a subset of hardware 
>>>>>> values
>>>>>> straight through.
>>>>>>
>>>>>> This will cause PVH dom0's not to use KPTI by default, and all dom0's 
>>>>>> not to
>>>>>> use VERW flushing by default,
>>>>> To avoid VERW, shouldn't you also expose SKIP_L1DFL?
>>>> SKIP_L1DFL is a software-only bit, specifically for nested virt.
>>>>
>>>> It is for Xen to tell an L1 hypervisor "you don't need to flush on
>>>> vmentry because I'm taking care of it".
>>> Or for a hypervisor underneath us to tell us, which we could then
>>> hand on to Dom0?
>> For dom0 to do what with?
>>
>> PV guests can't use the VMLAUNCH/VMRESUME instruction at all, and it is
>> not currently possible to configure nested virt for a PVH dom0 to use.
> Aren't they also using this on the exit-to-user-mode path, like we
> do on exit-to-PV? And in certain cases when idle?

MSR_FLUSH_CMD is used used for VMEntry.  This flushes the L1D cache, and
was to combat L1TF.  Native systems don't flush the L1D at all, and
invert PTEs instead as a *far* lower overhead mitigation.

Then MDS came along.  VERW is used to flush the uarch buffers.  This
needs doing in all return-to-guest contexts.

As VMEntry needs both, MSR_FLUSH_CMD's behaviour was extended to cover
both the L1D cache and uarch buffers, so software didn't have to arrange
for both.

Therefore, the overall mitigations are VERW on exit-to-PV, and
MSR_FLUSH_CMD on exit-to-HVM.


There is no current need for native setups to use MSR_FLUSH_CMD.  The
only reason we expose the MSR to HVM guests is for nested-virt.

~Andrew