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File xsa435-0-48.patch of Package xen

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
@@ -757,17 +757,6 @@ 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
@@ -271,7 +271,7 @@ 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 */

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