Re: [Xen-devel] [PATCH v2 0/3] x86: improve assisted tlb flush and use it in guest mode

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Fri, Jan 10, 2020 at 05:08:16PM +0100, Jan Beulich wrote:
> On 10.01.2020 17:04, Roger Pau Monne wrote:
> > Patch #2 is likely the most controversial one, as it changes the
> > implementation of assisted TLB flushes. I have to admit I haven't been
> > able to figure out why HVM guest context flushes issued a
> > flush_tlb_mask, and the commit introducing such behavior doesn't contain
> > a helpful commit message.
> 
> A shadow mode thing, maybe?

Hm, I could be wrong, but that flush doesn't seem to make sense for
shadow mode either.

If VPID/ASID is used, ticking it will drop all the guest caches, and
if VPID/ASID not used a vmexit/vmentry will clear the cache.
According to my reading of the Intel SDM this applies regardless of
whether HAP (EPT) is used or not.

The flush done by flush_tlb_mask is in root mode, and hence doesn't
affect the guest (non-root) caches when SVM/VTx is used.

Roger.

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