File xsa435-0-47.patch of Package xen.32200
From 4b2cdbfe766e5666e6754198946df2dc16f6a642 Mon Sep 17 00:00:00 2001
From: Andrew Cooper <andrew.cooper3@citrix.com>
Date: Wed, 17 May 2023 10:13:36 +0100
Subject: x86/cpu-policy: Advertise MSR_ARCH_CAPS to guests by default
With xl/libxl now able to control the policy bits for MSR_ARCH_CAPS, it is
safe to advertise to guests by default. In turn, we don't need the special
case to expose details to dom0.
This advertises MSR_ARCH_CAPS to guests on *all* Intel hardware, even if the
register content ends up being empty.
- Advertising ARCH_CAPS and not RSBA signals "retpoline is safe here and
everywhere you might migrate to". This is important because it avoids the
guest kernel needing to rely on model checks.
- Alternatively, levelling for safety across the Broadwell/Skylake divide
requires advertising ARCH_CAPS and RSBA, meaning "retpoline not safe on
some hardware you might migrate to".
On Cascade Lake and later hardware, guests can now see RDCL_NO (not vulnerable
to Meltdown) amongst others. This causes substantial performance
improvements, as guests are no longer applying software mitigations in cases
where they don't need to.
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
--- a/xen/arch/x86/cpu-policy.c
+++ b/xen/arch/x86/cpu-policy.c
@@ -769,17 +769,6 @@ void __init init_dom0_cpuid_policy(struc
if ( cpu_has_itsc )
p->extd.itsc = true;
- /*
- * Expose the "hardware speculation behaviour" bits of ARCH_CAPS to dom0,
- * so dom0 can turn off workarounds as appropriate. Temporary, until the
- * domain policy logic gains a better understanding of MSRs.
- */
- if ( is_hardware_domain(d) && cpu_has_arch_caps )
- {
- p->feat.arch_caps = true;
- p->arch_caps.raw = host_cpu_policy.arch_caps.raw;
- }
-
/* Apply dom0-cpuid= command line settings, if provided. */
if ( dom0_cpuid_cmdline )
{
--- a/xen/include/public/arch-x86/cpufeatureset.h
+++ b/xen/include/public/arch-x86/cpufeatureset.h
@@ -262,7 +262,7 @@ XEN_CPUFEATURE(CET_IBT, 9*32+20) /
XEN_CPUFEATURE(IBRSB, 9*32+26) /*A IBRS and IBPB support (used by Intel) */
XEN_CPUFEATURE(STIBP, 9*32+27) /*A STIBP */
XEN_CPUFEATURE(L1D_FLUSH, 9*32+28) /*S MSR_FLUSH_CMD and L1D flush. */
-XEN_CPUFEATURE(ARCH_CAPS, 9*32+29) /*! IA32_ARCH_CAPABILITIES MSR */
+XEN_CPUFEATURE(ARCH_CAPS, 9*32+29) /*!A IA32_ARCH_CAPABILITIES MSR */
XEN_CPUFEATURE(SSBD, 9*32+31) /*A MSR_SPEC_CTRL.SSBD available */
/* Intel-defined CPU features, CPUID level 0x00000007:1.eax, word 10 */