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Re: [Qemu-discuss] Disable, not hide cpu instructions/flags to the guest

From: Peter Maydell
Subject: Re: [Qemu-discuss] Disable, not hide cpu instructions/flags to the guest OS?
Date: Sun, 23 Jul 2017 21:26:11 +0100

On 23 July 2017 at 18:22, tukozaki <address@hidden> wrote:
> I tried launching the Qemu vm without kvm:
>     $ qemu-system-i386 -cpu pentium3,check <options, disk image>
> was slow as it should, but the same applications requiring sse2 still
> did run (with some patience).
> So I can't find how to run a client disabling the host cpu extensions.
> Maybe that simply is impossible with the current Qemu.

If the CPU you selected really does not have SSE2, then
the CPU emulation should generate the appropriate kind
of fault on attempts to execute those instructions.
If it doesn't, then that's a bug in the emulation.
It's not a very surprising kind of bug though, because
it's the kind of thing that nobody will notice in
normal operation. If you care for validation purposes
about this kind of thing you probably want to review
the QEMU code and/or exhaustively test all the instructions
you care about to be sure we don't have any lurking
bugs here.

(As you have discovered, you can't completely disable
the SSE instructions when using KVM -- this is because
the host CPU hardware does not support trapping to
the hypervisor (or otherwise faulting) on those

You might also like to consider a static analysis --
disassemble the binaries and scan them for uses of
SSE2/whatever instructions. This will obviously not
help for guest binaries which use JITs or other
runtime codegen, and you'll get false positives for
clever guest binaries that do runtime selection of
optimised routines based on CPU feature detection,
but it may still be useful as a fast initial pass,
and it has the advantage that it will detect SSE
instructions even if they're in a little-used part
of the code that your dynamic testing doesn't
happen to exercise.

-- PMM

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