Re: [PATCH v3 03/22] x86/xstate: re-size save area when CPUID policy changes

[Jan Beulich <jbeulich@xxxxxxxx>]

On 11.05.2021 18:41, Andrew Cooper wrote:
> On 03/05/2021 15:22, Jan Beulich wrote:
>>> Another consequence is that we need to rethink our hypercall behaviour. 
>>> There is no such thing as supervisor states in an uncompressed XSAVE
>>> image, which means we can't continue with that being the ABI.
>> I don't think the hypercall input / output blob needs to follow any
>> specific hardware layout.
> 
> Currently, the blob is { xcr0, xcr0_accum, uncompressed image }.
> 
> As we haven't supported any compressed states yet, we are at liberty to
> create a forward compatible change by logically s/xcr0/xstate/ and
> permitting an uncompressed image.
> 
> Irritatingly, we have xcr0=0 as a permitted state and out in the field,
> for "no xsave state".  This contributes a substantial quantity of
> complexity in our xstate logic, and invalidates the easy fix I had for
> not letting the HVM initpath explode.
> 
> The first task is to untangle the non-architectural xcr0=0 case, and to
> support compressed images.  Size parsing needs to be split into two, as
> for compressed images, we need to consume XSTATE_BV and XCOMP_BV to
> cross-check the size.

Not sure about the need to eliminate the xcr0=0 (or xstates=0) case.
Which isn't to say I'm opposed if you want to do so and it's not
overly intrusive.

> I think we also want a rule that Xen will always send compressed if it
> is using XSAVES (/XSAVEC in the interim?)

If this is sufficiently neutral to tool stack code, why not (albeit
I don't think there needs to be a "rule" - Xen should be free to
provide what it deems best, with consumers in the position to easily
recognize the format; similarly Xen should be consuming whatever it
gets handed, as long as that's valid state). Luckily the layout is
visible just through tool-stack-only interfaces.

>  We do not want to be working
> with uncompressed images at all, now that MPX is a reasonable sized hole
> in the middle.

They're together no larger (128 bytes) than the LWP hole right ahead
of them (at 0x340). I agree avoiding uncompressed format is worthwhile,
but perhaps quite a bit more so for systems with higher components
following unavailable even bigger ones.

Jan