Re: [Xen-devel] fpu_taskswitch adjustment proposal

[Keir Fraser <keir@xxxxxxx>]

On 15/06/2012 17:03, "Jan Beulich" <JBeulich@xxxxxxxx> wrote:

> While pv-ops so far doesn't care to eliminate the two trap-and-
> emulate CR0 accesses from the asm/xor.h save/restore
> operations, the legacy x86-64 kernel uses conditional clts()/stts()
> for this purpose. While looking into whether to extend this to the
> newly added (in 3.5) AVX operations there I realized that this isn't
> fully correct: It doesn't properly nest inside a kernel_fpu_begin()/
> kernel_fpu_end() pair (as it will stts() at the end no matter what
> the original state of CR0.TS was).
> 
> In order to not introduce completely new hypercalls to overcome
> this (fpu_taskswitch isn't really extensible on its own), I'm
> considering to add a new VM assist, altering the fpu_taskswitch
> behavior so that it would return an indicator whether any change
> to the virtual CR0.TS was done. That way, the kernel can
> implement a true save/restore cycle here.

It should be possible for the guest kernel to track its CR0.TS setting
shouldn't it? It gets modified only via a few paravirt hooks, and implicitly
cleared on #NM.

 -- Keir

> In order to allow the kernel to run on older hypervisors without
> extra conditionals (behaving the same way as it does currently,
> i.e. with the incorrect nesting), the return value 0 (which the
> hypercall currently always returns) would need to be used to
> indicate that the bit got actually flipped (such that on an old
> hypervisor an updated kernel would always think that
> something needs to be restored).
> 
> Would that be an acceptable solution? Can someone think of
> other ways to deal with this? (And - is pv-ops interested in
> eliminating the two CR0 access emulations in what is supposed
> to be a fast path?)
> 
> Jan
> 
> 
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