Re: [Xen-devel] [PATCH 09/15] x86/emul: Avoid raising faults behind the emulators back

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 23/11/16 16:31, Tim Deegan wrote:
> At 15:38 +0000 on 23 Nov (1479915532), Andrew Cooper wrote:
>> Introduce a new x86_emul_pagefault() similar to x86_emul_hw_exception(), and
>> use this instead of hvm_inject_page_fault() from emulation codepaths.
>>
>> Replace one hvm_inject_hw_exception() in the shadow emulation codepaths.
>>
>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>> NOTE: this is a functional change for the shadow code, as a #PF previously
>> raised properly with the guest will now cause X86EMUL_UNHANDLABLE. It is my
>> understanding after a discusion with Tim that this is ok, but I haven't done
>> extenstive testing yet.
> Do you plan to?  I think this is indeed OK, but there may be some edge
> case, e.g. an instruction that writes to both the current top-level
> pagetable (which can't be unshadowed) and an unmapped virtual address.
> That ought to raise #PF in the guest but might now spin retrying?

That is a devious corner case.  I take it you have been there before?

The more I think about these changes, the more I think that the shadow
code would be better by selectively looking a pending event, injecting
pagefaults, but rejecting and retrying if any other event shows up.

~Andrew

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