Re: [Xen-devel] [PATCH 2/2] x86: use invpcid to do global flushing

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Mon, Mar 05, 2018 at 06:24:37AM -0700, Jan Beulich wrote:
> >>> On 05.03.18 at 14:11, <jgross@xxxxxxxx> wrote:
> > On 05/03/18 13:57, Andrew Cooper wrote:
> >> When we start using PCID for user mappings, then we don't need them to
> >> be global, at which point we can require/expect that the only global
> >> mappings are hypervisor ones which we expect to remain correct across a
> >> write to cr3.  However, if we do this, then we need to use a bit other
> >> than PAGE_GLOBAL to signify guest user mappings.
> >> 
> >> I think this is doable, but I don't think it is going to be trivial to
> >> get correct.
> > 
> > Why would we want to keep any global mappings at all? What are they good
> > for? Today the only case I could find where they make sense at all is
> > for 64-bit pv-guests to keep hypervisor mappings in the TLB when the
> > guest is switching between user and kernel mode.
> 
> Hypervisor and guest user mappings, that is.
> 

I'm not sure I understand the rationale behind global guest mappings. Is
it to keep guest user mappings when switching to guest kernel mode?

(A normal kernel like Linux makes global kernel mappings, but obv. we
can't do that in 64bit PV guests because kernel is also in ring3.)

Wei.

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