[PATCH v4 27/30] x86/svm: restore guest debug registers in svm_do_resume()

[George Dunlap <gwd@xxxxxxxxxxxxxx>]

svm_ctxt_switch_to() restores the guest's debug registers when its DR7
enables any of them, which also updates the VMCB.  Do it in
svm_do_resume() instead, which follows it whenever the vCPU is
scheduled in.

svm_do_resume() also runs when a vCPU keeps running on its pCPU
(continue_running()), without svm_ctxt_switch_to().  Where the guest's
debug registers are already loaded, the restore does nothing:
flag_dr_dirty records that they are, and only svm_save_dr() clears it,
from svm_ctxt_switch_from().

This prepares for moving VMCBs off always-mapped memory:
svm_ctxt_switch_to() runs before write_ptbase(), where the incoming
vCPU's VMCB will not be mapped.

Not yet run with a guest using debug registers.

Assisted-by: Claude Code:claude-fable-5, Claude Code:claude-opus-5-5
Signed-off-by: George Dunlap <gwd@xxxxxxxxxxxxxx>
---
Changes in v4:
- New in this version.
---
 xen/arch/x86/hvm/svm/svm.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/hvm/svm/svm.c b/xen/arch/x86/hvm/svm/svm.c
index 4e19df9865..3de8da7ea3 100644
--- a/xen/arch/x86/hvm/svm/svm.c
+++ b/xen/arch/x86/hvm/svm/svm.c
@@ -955,8 +955,6 @@ static void cf_check svm_ctxt_switch_to(struct vcpu *v)
      */
     disable_each_ist(per_cpu(idt, cpu));
 
-    svm_restore_dr(v);
-
     svm_tsc_ratio_load(v);
 
     if ( cpu_has_msr_tsc_aux )
@@ -980,6 +978,8 @@ static void noreturn cf_check svm_do_resume(void)
     bool vcpu_guestmode = false;
     struct vlapic *vlapic = vcpu_vlapic(v);
 
+    svm_restore_dr(v);
+
     if ( nestedhvm_enabled(v->domain) && nestedhvm_vcpu_in_guestmode(v) )
         vcpu_guestmode = 1;
 
-- 
2.55.0