Re: [Xen-devel] [PATCH v3 7/7] xen/x86: use PCID feature

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 26.03.18 at 12:29, <jgross@xxxxxxxx> wrote:
> On 26/03/18 12:13, Jan Beulich wrote:
>>>>> On 26.03.18 at 10:55, <jgross@xxxxxxxx> wrote:
>>> I can change the scheme to use different values for guest PCIDs
>>> with XPTI on, of course. Are you fine with:
>>>
>>> - XPTI off: PCID 0 = kernel, PCID 1 = user
>>> - XPTI on:  PCID 0 = kernel/Xen, PCID 1 = user/Xen,
>>>             PCID 2 = kernel/guest, PCID 3 = user/guest
>> 
>> Yes, that would fit the first variant I've described. I take it you
>> prefer not to avoid PCID 0 altogether when PCIDs are enabled -
>> is there a particular reason?
> 
> Yes. As written in the comment in flush_area_local() I can't be sure
> whether the current address space is that of a domain with XPTI
> enabled (the idle domain could be "current"). So I need to always
> flush with PCID 0 and with the possible PCID values for a XPTI domain.
> When using PCID 0 for XPTI as well I'll need 4 INVPCIDs, while when
> avoiding it I'd need 5 (at least when current == idle).

I see. Which makes me wonder whether a suitable combination
of INVLPG (to get rid of global entries) and INVPCID couldn't be
used instead. For example, you may be able to replace the
INVPCID for the active PCID by INVLPG (without needing to
know who "current" is).

Jan


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