[Bug 1864536] Re: Support for XSAVES intel instructions in QEMU

From: Thomas Huth <1864536@bugs.launchpad.net>
To: qemu-devel@nongnu.org
Subject: [Bug 1864536] Re: Support for XSAVES intel instructions in QEMU
Date: Thu, 06 May 2021 05:38:15 -0000	[thread overview]
Message-ID: <162027949599.6519.11247163377551789172.malone@wampee.canonical.com> (raw)
In-Reply-To: 158257353920.30257.1886439598745813406.malonedeb@wampee.canonical.com

The QEMU project is currently moving its bug tracking to another system.
For this we need to know which bugs are still valid and which could be
closed already. Thus we are setting older bugs to "Incomplete" now.

If you still think this bug report here is valid, then please switch
the state back to "New" within the next 60 days, otherwise this report
will be marked as "Expired". Or please mark it as "Fix Released" if
the problem has been solved with a newer version of QEMU already.

Thank you and sorry for the inconvenience.

** Changed in: qemu
       Status: New => Incomplete

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https://bugs.launchpad.net/bugs/1864536

Title:
  Support for XSAVES intel instructions in QEMU

Status in QEMU:
  Incomplete

Bug description:
  Dear QEMU developers,

  I am running Hyper-V on qemu+kvm. During it initialization, it checks
  for XSAVES support: first it executes CPUID with EAX = 0xd and ECX = 1
  and looks at bit 3 in the returned value of EAX (Supports
  XSAVES/XRSTORS and IA32_XSS [1]), and then it reads the MSR
  IA32_VMX_PROCBASED_CTLS2 (index 0x48B) and looks at bit 20 (Enable
  XSAVES/XSTORS [2]). If CPUID shows that XSAVES is supported and the
  bit is not enabled in the MSR, Hyper-V decides to fail and stops its
  initialization. It used to work until last spring/summer where
  something might have changed in either KVM or QEMU.

  It seems that KVM sets the correct flags (in CPUID and the MSR) when the host CPU supports XSAVES. In QEMU, based on comments in target/i386/cpu.c it seems that XSAVES is not added in
  builtin_x86_defs[].features[FEAT_VMX_SECONDARY_CTLS] because it might break live migration. Therefore, when setting the MSR for the vcpu, QEMU is masking off the feature.

  I have tested two possible solutions:
  - adding the flag in .features[FEAT_VMX_SECONDARY_CTLS]
  - removing the support of the instruction in feature_word_info[FEAT_XSAVE].feat_names

  Both solutions work and Hyper-v is happily running. I can provide a
  patch for the solution you might consider applying. Otherwise, is
  there a better way to fix the issue?

  Qemu version: 4.2.0
  Kernel version: 5.5.4
  Qemu command: https://gist.github.com/0xabe-io/b4d797538e2160252addc1d1d64738e2

  Many thanks,
  Alexandre

  Ref:
  [1] Intel SDM Volume 2A, chapter 3, page 196
  [2] Intel SDM Volume 3C, chapter 24, page 11

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