[PATCH v3] x86/amd: do not expose HWCR.TscFreqSel to guests

Roger Pau Monne posted 1 patch 1 week ago
Patches applied successfully (tree, apply log)
git fetch https://gitlab.com/xen-project/patchew/xen tags/patchew/20230914145436.92358-1-roger.pau@citrix.com
xen/arch/x86/msr.c | 3 +--
1 file changed, 1 insertion(+), 2 deletions(-)
[PATCH v3] x86/amd: do not expose HWCR.TscFreqSel to guests
Posted by Roger Pau Monne 1 week ago
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 <solene@openbsd.org>
Link: https://github.com/QubesOS/qubes-issues/issues/8502
Fixes: 14b95b3b8546 ('x86/AMD: expose HWCR.TscFreqSel to guests')
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
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;