Re: [Xen-devel] [PATCH v2 4/6] xen/x86: disable global pages for domains with XPTI active

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 09.03.18 at 06:23, <kevin.tian@xxxxxxxxx> wrote:
>>  From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
>> Sent: Thursday, March 8, 2018 9:39 PM
>> 
>> > --- a/xen/include/asm-x86/domain.h
>> > +++ b/xen/include/asm-x86/domain.h
>> > @@ -622,7 +622,8 @@ unsigned long pv_guest_cr4_fixup(const struct
>> vcpu *, unsigned long guest_cr4);
>> >              X86_CR4_SMAP | X86_CR4_OSXSAVE |                \
>> >              X86_CR4_FSGSBASE))                              \
>> >        | ((v)->domain->arch.vtsc ? X86_CR4_TSD : 0))         \
>> > -     & ~X86_CR4_DE)
>> > +     & ~(X86_CR4_DE |                                       \
>> > +         ((v)->domain->arch.pv_domain.xpti ? X86_CR4_PGE : 0)))
>> 
>> With this you manage to turn off global pages when switching to
>> a PV vCPU. But I can't see how you turn global pages back on when
>> switching away from it. I can see they would be turned back on e.g.
>> on the first entry to a VMX guest, but how about an SVM one? And
>> how about the time between switching away from the PV vCPU and
>> that VM entry? Granted all flushes are global ones right now, but
>> that should change with the modification here: If you look back at
>> 4.2 code, you'll see that FLUSH_TLB was handled differently in that
>> case, retaining Xen's global mappings. Any flush IPI not requesting
>> global pages to be flushed could then leave intact Xen's own TLB
>> entries, which takes as a prereq that CR4.PGE gets turned back on
>> earlier.
> 
> btw does PGE really matter regarding to entry to HVM guest? Xen's 
> mappings are either all flushed (vpid disabled) or all sustained
> (vpid enabled) at VM entries, regardless of global setting. then if 
> PGE is anyway turned off for PV and doesn't matter for HVM is it 
> still useful to keep it turned on between switches?

Well, yes indeed, but that's only half of where we may want to
retain them (if possible). The other half is the time between
context switching in a HVM vCPU and the subsequent VM entry (or
to be precise the point in time when interrupts get turned off
before that VM entry, as that's where flush IPIs can still arrive).

Jan


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