[Xen-devel] [PATCH 10/11] x86/vmx: Work around VMEntry failure when Single Stepping in an STI shadow

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

See the code comment for the details.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Jun Nakajima <jun.nakajima@xxxxxxxxx>
CC: Kevin Tian <kevin.tian@xxxxxxxxx>

Jun/Kevin: This workaround is as suggested by Gil, and there is expected to be
an SDM update discussing the corner case.

Note that, like elsewhere dealing with eflags.tf, this is probably buggy in
combination with MSR_DEBUGCTL.BTF.  I'll untangle the BTF swamp at some later
point.
---
 xen/arch/x86/hvm/vmx/vmx.c | 30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
index 82ef3aa..58ff8c7 100644
--- a/xen/arch/x86/hvm/vmx/vmx.c
+++ b/xen/arch/x86/hvm/vmx/vmx.c
@@ -1794,6 +1794,36 @@ static void vmx_inject_event(const struct x86_event 
*event)
         write_debugreg(6, merge_dr6(read_debugreg(6), event->pending_dbg,
                                     curr->domain->arch.cpuid->feat.rtm));
 
+        /*
+         * Work around SS/STI vmentry bug.
+         *
+         * If kernel code is single stepping itself and executes an STI
+         * instruction resulting in an STI shadow, a vmexit occurs due to #DB
+         * interception, but the vmentry fails due to a failed consistency
+         * check.  (Hardware comes to the conclusion that there should be a
+         * pending debug exception, but doesn't account for the pending #DB in
+         * VMENTRY_INTR_INFO.)
+         *
+         * Manually adjust the pending debug exception field to mark BS as
+         * pending, which satisfies the consistency check and allows the
+         * vmentry to succeed.
+         */
+        if ( unlikely(regs->eflags & X86_EFLAGS_TF) )
+        {
+            unsigned long int_info;
+
+            __vmread(GUEST_INTERRUPTIBILITY_INFO, &int_info);
+
+            if ( int_info & VMX_INTR_SHADOW_STI )
+            {
+                unsigned long pending_dbg;
+
+                __vmread(GUEST_PENDING_DBG_EXCEPTIONS, &pending_dbg);
+                __vmwrite(GUEST_PENDING_DBG_EXCEPTIONS,
+                          pending_dbg | X86_DR6_BS);
+            }
+        }
+
         if ( !nestedhvm_vcpu_in_guestmode(curr) ||
              !nvmx_intercepts_exception(curr, TRAP_debug, _event.error_code) )
         {
-- 
2.1.4


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