[xen staging] x86/intel: Expose MSR_ARCH_CAPS to dom0

[patchbot@xxxxxxx]

commit e46474278a0e87e2b32ad5dd5fc20e8d2cb0688b
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Tue Apr 10 16:25:40 2018 +0100
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Mon Aug 31 13:43:26 2020 +0100

    x86/intel: Expose MSR_ARCH_CAPS to dom0
    
    The overhead of (the lack of) MDS_NO alone has been measured at 30% on some
    workloads.  While we're not in a position yet to offer MSR_ARCH_CAPS 
generally
    to guests, dom0 doesn't migrate, so we can pass a subset of hardware values
    straight through.
    
    This will cause PVH dom0's not to use KPTI by default, and all dom0's not to
    use VERW flushing by default, and to use eIBRS in preference to retpoline on
    recent Intel CPUs.
    
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/arch/x86/cpuid.c |  8 ++++++++
 xen/arch/x86/msr.c   | 16 ++++++++++++++++
 2 files changed, 24 insertions(+)

diff --git a/xen/arch/x86/cpuid.c b/xen/arch/x86/cpuid.c
index 63a03ef1e5..4b424fac95 100644
--- a/xen/arch/x86/cpuid.c
+++ b/xen/arch/x86/cpuid.c
@@ -719,6 +719,14 @@ int init_domain_cpuid_policy(struct domain *d)
     if ( d->disable_migrate )
         p->extd.itsc = cpu_has_itsc;
 
+    /*
+     * 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) && boot_cpu_has(X86_FEATURE_ARCH_CAPS) )
+        p->feat.arch_caps = true;
+
     d->arch.cpuid = p;
 
     recalculate_cpuid_policy(d);
diff --git a/xen/arch/x86/msr.c b/xen/arch/x86/msr.c
index c3862033eb..a478b91f23 100644
--- a/xen/arch/x86/msr.c
+++ b/xen/arch/x86/msr.c
@@ -130,6 +130,22 @@ int init_domain_msr_policy(struct domain *d)
     if ( !opt_dom0_cpuid_faulting && is_control_domain(d) && is_pv_domain(d) )
         mp->platform_info.cpuid_faulting = false;
 
+    /*
+     * 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) && boot_cpu_has(X86_FEATURE_ARCH_CAPS) )
+    {
+        uint64_t val;
+
+        rdmsrl(MSR_ARCH_CAPABILITIES, val);
+
+        mp->arch_caps.raw = val &
+            (ARCH_CAPS_RDCL_NO | ARCH_CAPS_IBRS_ALL | ARCH_CAPS_RSBA |
+             ARCH_CAPS_SSB_NO | ARCH_CAPS_MDS_NO | ARCH_CAPS_TAA_NO);
+    }
+
     d->arch.msr = mp;
 
     return 0;
--
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