[Xen-devel] [PATCHv3 0/3] x86: workaround inability to fully restore FPU state

[David Vrabel <david.vrabel@xxxxxxxxxx>]

This series extends the workaround for the inability for some x86 CPUs
to fully restore the FPU exception state (64-bit FIP/FDP and FCS/FDS).

Toolstack (or the guest) may override the default behaviour to always
do a 32-bit save/restore.

Running Microsoft's Driver Verifier continues to work in a 32-bit
Windows guest and (if HVM_PARAM_X87_FIP_WIDTH is set to 4) now works
in a 64-bit Windows guest.

Changes in v3:

- Use a random value to poison FIP when checking if the hardware
  writes it.
- Skip checking for FIP updates if 8 is always requested.
- Only update word size if FP state was requested.
- Improve comments a bit.

Changes in v2:

- Improve xsave()'s detection of whether the hardware updated FIP/FDP.
- Leave 64-bit PV guests in auto mode.
- Don't automatically set FIP width for Windows guests -- it's safer
  to leave auto-mode on and leave it up to the admin to enable the
  mode when running Driver Verifier.
- Use a HVM param to change the default.

David



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