Re: [Xen-devel] [PATCH RFC V4 1/5] xen: Emulate with no writes

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 05.08.14 at 17:16, <rcojocaru@xxxxxxxxxxxxxxx> wrote:
> So if I understand this code correctly, hvmemul_rep_ins() performs a few
> checks, and then calls hvmemul_do_pio(), which ends up calling
> hvmemul_do_io(), which seems to discard the write rather unceremoniously
> for weird-sized accesses. This would seem to roughly correspond to just
> returning X86EMUL_UNHANDLEABLE from hvmemul_rep_ins() for that special
> case (with no MMIO code executed).
> 
> Did I misunderstand something?

The main issue is that as long as these functions only act as a backend
to the x86 instruction emulator, "weird size accesses" just can't happen.
I.e. the respective check is just a guard against careless future code
additions. And for such bad cases it is of course appropriate to drop
the entire effect of the instruction - this is largely like a fault happing on
it. And quite different from your case - you want the instruction to
behave normally _except_ for the memory write as I understood so far
(albeit it still escapes me how in the end correct behavior can result).

Furthermore X86EMUL_UNHANDLEABLE generally results in a fault to be
injected into the guest, i.e. such paths aren't suitable for your purposes
iiuc.

Jan


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