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

[Jan Beulich <jbeulich@xxxxxxxx>]

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?

Jan