Re: [Xen-devel] [PATCH v6 4/6] x86/tlb: introduce a flush guests TLB flag

[Jan Beulich <jbeulich@xxxxxxxx>]

On 03.03.2020 18:20, Roger Pau Monne wrote:
> Introduce a specific flag to request a HVM guest TLB flush, which is
> an ASID/VPID tickle that forces a guest linear to guest physical TLB
> flush for all HVM guests.
> 
> This was previously unconditionally done in each pre_flush call, but
> that's not required: HVM guests not using shadow don't require linear
> TLB flushes as Xen doesn't modify the guest page tables in that case
> (ie: when using HAP). Note that shadow paging code already takes care
> of issuing the necessary flushes when the shadow page tables are
> modified.
> 
> In order to keep the previous behavior modify all shadow code TLB
> flushes to also flush the guest linear to physical TLB, in order to
> keep the previous behavior. I haven't looked at each specific shadow
> code TLB flush in order to figure out whether it actually requires a
> guest TLB flush or not, so there might be room for improvement in that
> regard.

I'm still missing a discussion of the safety of the removal of the
tickle from the switch_cr3_cr4() path: I don't think it can be
excluded offhand that shadow code somewhere relies on this happening
as a side effect of write_ptbase().

Jan

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