[Bug 1904317] [NEW] cpu feature selection is not affected to guest 's cpuid with whpx

[Takumi Nakamura]

Public bug reported:

On windows with -accel whpx, "-cpu" is ignored without any messages.
Guest recognizes features as same as host's.

Confirmed on v5.2.0-rc1.

I suggest qemu may do,

- Warn with incompatible -cpu options were given.
- Enhance cpuid handling.

Background:
I was investigated mmio and block copy issue in Linux kernel.
I met a problem that Linux went ill for touching mmio with whpx. (not with tcg)
I suspect erms(enhanced rep movs) might trigger.
I tried to mask erms on qemu with -feature,erms, but it was ineffective.

At last, I disabled erms manually, to tweak whpx-all.c to mask erms in
cpuid.

FYI, qemu with whpx from/to mmio, "rep movsb" does byte access regardless of 
erms.
Linux kernel tends to choose not "rep movsq" but "rep movsb" with erms.

** Affects: qemu
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1904317

Title:
  cpu feature selection is not affected to guest 's cpuid with whpx

Status in QEMU:
  New

Bug description:
  On windows with -accel whpx, "-cpu" is ignored without any messages.
  Guest recognizes features as same as host's.

  Confirmed on v5.2.0-rc1.

  I suggest qemu may do,

  - Warn with incompatible -cpu options were given.
  - Enhance cpuid handling.

  Background:
  I was investigated mmio and block copy issue in Linux kernel.
  I met a problem that Linux went ill for touching mmio with whpx. (not with 
tcg)
  I suspect erms(enhanced rep movs) might trigger.
  I tried to mask erms on qemu with -feature,erms, but it was ineffective.

  At last, I disabled erms manually, to tweak whpx-all.c to mask erms in
  cpuid.

  FYI, qemu with whpx from/to mmio, "rep movsb" does byte access regardless of 
erms.
  Linux kernel tends to choose not "rep movsq" but "rep movsb" with erms.

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