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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 05/03/18 13:31, Wei Liu wrote:
> 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?

Yes - it is a performance optimisation so on guest user => guest kernel
context switch, guest user mappings are still present in the TLB.

~Andrew

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