Re: [Xen-devel] [PATCH] x86/HVM: extend LAPIC shortcuts around P2M lookups

[Tim Deegan <tim@xxxxxxx>]

At 15:27 +0100 on 01 Aug (1406903251), Jan Beulich wrote:
> >>> On 01.08.14 at 15:43, <JBeulich@xxxxxxxx> wrote:
> > ... to all internally handled MMIO regions. It is in particular the
> > HPET page that, e.g. on Windows Server 2012 R2, can get heavily
> > accessed, and hence avoiding the unnecessary lookups is rather
> > beneficial (in the reported case a 40+-vCPU guest would previously not
> > have booted at all while with hvm_hap_nested_page_fault() shortcut
> > alone it was able to boot up in 18 minutes [i.e. still room for
> > improvement]).
> 
> Btw., while I expect the second shortcut to also help a little (I was
> only able to functionality test it, as I don't have a big enough system
> around to meaningfully test that big a guest), while going through
> all the pCPU-s' stack trace snapshots it occurred to me that for
> hvm_hap_nested_page_fault()-induced MMIO emulation it is in many
> cases quite pointless to "manually" do the VA->GPA translation, since
> the handler already gets passed the offending GPA. Of course some
> care would need to be taken to e.g. not use this on instructions
> having more than one memory operand, or where the memory
> operand crosses page boundaries, but all the information needed
> for this would be available after decoding the instruction, i.e. well
> in time before evaluating instruction operands. Am I overlooking
> any other aspect making such an optimization unsafe?

If Xen does its own instruction fetch and decode, then we have to be
careful about reusing any state from the original exit because of
self-modifying code.  (And yes, that is a serious concern -- I once
spent months trying to debug occasional memory corruption in the
self-modifying license-enforcement code on a system stress test
utility.)

So it would be OK to reuse the GPA from the exit if we could verify
that the GVA we see is the same as the original fault (since there can't
have been a TLB flush).  But IIRC the exit doesn't tell us the
original GVA. :(

Tim.

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