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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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?

Jan


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