[PATCH v6 43/44] x86/domain: restore FPU state after switching to the new page tables

[George Dunlap <gwd@xxxxxxxxxxxxxx>]

__context_switch() restores the incoming vCPU's FPU state
(vcpu_restore_fpu(n)) while still running on the outgoing vCPU's page
tables, before write_ptbase(n).  That dereferences n's XSAVE area
through whatever address it happens to have in p's address space --
fine today, when the area is globally mapped, but not once guest XSAVE
state moves to a per-vCPU mapping that need not exist in another vCPU's
address space (upcoming Address Space Isolation work).

Move the eager restore to just after write_ptbase(n), so it runs with
n's page tables loaded.  The XCR0/XSS setup stays where it is (it
programs control registers / an MSR, independent of the page tables,
and XSS must be set before the supervisor-state xrstor).  Nothing
between the save and the new restore point touches guest FPU state:
the ctxt_switch->to() hooks, psr and GDT handling are all FPU-free,
and Xen is built without SSE.

No functional change with the current global XSAVE-area mapping.

Assisted-by: Claude Code:claude-opus-4-8
Signed-off-by: George Dunlap <gwd@xxxxxxxxxxxxxx>
---
Changes in v5:
- New in this version.
---
 xen/arch/x86/domain.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
index 66d71ec41b..8222ad9e2a 100644
--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -2267,7 +2267,6 @@ static void __context_switch(void)
             if ( cpu_has_xsaves && is_hvm_vcpu(n) )
                 set_msr_xss(n->arch.msrs->xss.raw);
         }
-        vcpu_restore_fpu(n);
         nd->arch.ctxt_switch->to(n);
     }
 
@@ -2283,7 +2282,10 @@ static void __context_switch(void)
     write_ptbase(n);
 
     if ( !is_idle_domain(nd) )
+    {
         memcpy(stack_regs, vcpu_user_regs(n), CTXT_SWITCH_STACK_BYTES);
+        vcpu_restore_fpu(n);
+    }
 
 #ifdef CONFIG_PV
     /* Prefetch the VMCB if we expect to use it later in the context switch */
-- 
2.56.0