OpenBSD 7.3 will unconditionally access HWCR if the TSC is reported as Invariant, and it will then attempt to also unconditionally access PSTATE0 if HWCR.TscFreqSel is set (currently the case on Xen).
The motivation for exposing HWCR.TscFreqSel was to avoid warning messages from Linux. It has been agreed that Linux should be changed instead to not complaint about missing HWCR.TscFreqSel when running virtualized. The relation between HWCR.TscFreqSel and PSTATE0 is not clearly written down in the PPR, but it's natural for OSes to attempt to fetch the P0 frequency if the TSC is stated to increment at the P0 frequency. Exposing PSTATEn (PSTATE0 at least) with all zeroes is not a suitable solution because the PstateEn bit is read-write, and OSes could legitimately attempt to set PstateEn=1 which Xen couldn't handle. Furthermore, the TscFreqSel bit is model specific and was never safe to expose like this in the first place. At a minimum it should have had a toolstack adjustment to know not to migrate such a VM. Therefore, simply remove the bit. Note the HWCR itself is an architectural register, and does need to be accessible by the guest. Since HWCR contains both architectural and non-architectural bits, going forward care must be taken to assert the exposed value is correct on newer CPU families. Reported-by: Solène Rapenne <[email protected]> Link: https://github.com/QubesOS/qubes-issues/issues/8502 Fixes: 14b95b3b8546 ('x86/AMD: expose HWCR.TscFreqSel to guests') Signed-off-by: Roger Pau Monné <[email protected]> Reviewed-by: Andrew Cooper <[email protected]> --- Changes since v2: - Adjust commit message. --- xen/arch/x86/msr.c | 3 +-- 1 file changed, 1 insertion(+), 2 deletions(-) diff --git a/xen/arch/x86/msr.c b/xen/arch/x86/msr.c index 3f0450259cdf..c33dc78cd8f6 100644 --- a/xen/arch/x86/msr.c +++ b/xen/arch/x86/msr.c @@ -240,8 +240,7 @@ int guest_rdmsr(struct vcpu *v, uint32_t msr, uint64_t *val) case MSR_K8_HWCR: if ( !(cp->x86_vendor & (X86_VENDOR_AMD | X86_VENDOR_HYGON)) ) goto gp_fault; - *val = get_cpu_family(cp->basic.raw_fms, NULL, NULL) >= 0x10 - ? K8_HWCR_TSC_FREQ_SEL : 0; + *val = 0; break; case MSR_VIRT_SPEC_CTRL: -- 2.42.0
