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KVM: x86: Virtualize HWCR.TscFreqSel[bit 24]

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On certain CPUs, Linux guests expect HWCR.TscFreqSel[bit 24] to be
set. If it isn't set, they complain:
	[Firmware Bug]: TSC doesn't count with P0 frequency!

Allow userspace (and the guest) to set this bit in the virtual HWCR to
eliminate the above complaint.

Allow the guest to write the bit even though its is R/O on *some* CPUs.
Like many bits in HWRC, TscFreqSel is not architectural at all. On Family
10h[1], it was R/W and powered on as 0. In Family 15h, one of the "changes
relative to Family 10H Revision D processors[2] was:

  • MSRC001_0015 [Hardware Configuration (HWCR)]:
  • Dropped TscFreqSel; TSC can no longer be selected to run at NB P0-state.

Despite the "Dropped" above, that same document later describes
HWCR[bit 24] as follows:

  TscFreqSel: TSC frequency select. Read-only. Reset: 1. 1=The TSC
  increments at the P0 frequency

If the guest clears the bit, the worst case scenario is the guest will be
no worse off than it is today, e.g. the whining may return after a guest
clears the bit and kexec()'s into a new kernel.

[1] https://www.amd.com/content/dam/amd/en/documents/archived-tech-docs/programmer-references/31116.pdf
[2] https://www.amd.com/content/dam/amd/en/documents/archived-tech-docs/programmer-references/42301_15h_Mod_00h-0Fh_BKDG.pdf,
Signed-off-by: Jim Mattson <jmattson@google.com>
Link: https://lore.kernel.org/r/20230929230246.1954854-3-jmattson@google.com
[sean: elaborate on why the bit is writable by the guest]
Signed-off-by: Sean Christopherson <seanjc@google.com>
This commit is contained in:
Jim Mattson 2023-09-29 16:02:45 -07:00 committed by Sean Christopherson
parent 598a790fc2
commit 8b0e00fba9
1 changed files with 6 additions and 2 deletions
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8
arch/x86/kvm/x86.c
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@ -3790,8 +3790,12 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
data &= ~(u64)0x100; /* ignore ignne emulation enable */
data &= ~(u64)0x8; /* ignore TLB cache disable */
/* Handle McStatusWrEn */
if (data & ~BIT_ULL(18)) {
/*
* Allow McStatusWrEn and TscFreqSel. (Linux guests from v3.2
* through at least v6.6 whine if TscFreqSel is clear,
* depending on F/M/S.
*/
if (data & ~(BIT_ULL(18) | BIT_ULL(24))) {
kvm_pr_unimpl_wrmsr(vcpu, msr, data);
return 1;
}