Re: [Xen-devel] [PATCH 3/4] x86emul: correct handling of FPU insns faulting on memory write

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 13/03/17 11:05, Jan Beulich wrote:
> When an FPU instruction with a memory destination fails during the
> memory write, it should not affect FPU register state. Due to the way
> we emulate FPU (and SIMD) instructions, we can only guarantee this by
> - backing out changes to the FPU register state in such a case or
> - doing a descriptor read and/or page walk up front, perhaps with the
>   stubs accessing the actual memory location then.
> The latter would require a significant change in how the emulator does
> its guest memory accessing, so for now the former variant is being
> chosen.
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

> ---
> Note that the state save overhead (unless state hadn't been loaded at
> all before, which should only be possible if a guest is fiddling with
> the instruction stream under emulation) is taken for every FPU insn
> hitting the emulator. We could reduce this to just the ones writing to
> memory, but that would involve quite a few further changes and
> resulting code where even more code paths need to match up with one
> another.

An FPU save isn't the slow part of hitting emulation.  I don't expect
this will be a meaningful overhead.

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