[xen staging] x86emul: allow ->write_msr() to distinguish origins of writes

[patchbot@xxxxxxx]

commit 163e49e4196098ec186bb584c49e7c4b727f94e7
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Mon Feb 2 09:05:45 2026 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Mon Feb 2 09:05:45 2026 +0100

    x86emul: allow ->write_msr() to distinguish origins of writes
    
    Only explicit writes are subject to e.g. the checking of the MSR intercept
    bitmap, which extends to VM-event's hvm_monitor_msr(). Indicate, by way of
    a new boolean parameter, to the hook functions which variant it is.
    
    Fixes: 6eb43fcf8a0b ("x86emul: support SWAPGS")
    Reported-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Reviewed-by: Jason Andryuk <jason.andryuk@xxxxxxx>
    Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
 tools/fuzz/x86_instruction_emulator/fuzz-emul.c | 3 ++-
 xen/arch/x86/hvm/emulate.c                      | 8 +++++---
 xen/arch/x86/pv/emul-priv-op.c                  | 3 ++-
 xen/arch/x86/x86_emulate/0f01.c                 | 6 +++---
 xen/arch/x86/x86_emulate/x86_emulate.c          | 2 +-
 xen/arch/x86/x86_emulate/x86_emulate.h          | 8 +++++---
 6 files changed, 18 insertions(+), 12 deletions(-)

diff --git a/tools/fuzz/x86_instruction_emulator/fuzz-emul.c 
b/tools/fuzz/x86_instruction_emulator/fuzz-emul.c
index 28c4fb13c5..2b9b72df35 100644
--- a/tools/fuzz/x86_instruction_emulator/fuzz-emul.c
+++ b/tools/fuzz/x86_instruction_emulator/fuzz-emul.c
@@ -569,7 +569,8 @@ static int fuzz_read_msr(
 static int fuzz_write_msr(
     unsigned int reg,
     uint64_t val,
-    struct x86_emulate_ctxt *ctxt)
+    struct x86_emulate_ctxt *ctxt,
+    bool explicit)
 {
     struct fuzz_state *s = ctxt->data;
     struct fuzz_corpus *c = s->corpus;
diff --git a/xen/arch/x86/hvm/emulate.c b/xen/arch/x86/hvm/emulate.c
index d56ef02baf..86b1f75356 100644
--- a/xen/arch/x86/hvm/emulate.c
+++ b/xen/arch/x86/hvm/emulate.c
@@ -1705,7 +1705,8 @@ static int cf_check hvmemul_write_io_discard(
 static int cf_check hvmemul_write_msr_discard(
     unsigned int reg,
     uint64_t val,
-    struct x86_emulate_ctxt *ctxt)
+    struct x86_emulate_ctxt *ctxt,
+    bool explicit)
 {
     return X86EMUL_OKAY;
 }
@@ -2427,9 +2428,10 @@ static int cf_check hvmemul_read_msr(
 static int cf_check hvmemul_write_msr(
     unsigned int reg,
     uint64_t val,
-    struct x86_emulate_ctxt *ctxt)
+    struct x86_emulate_ctxt *ctxt,
+    bool explicit)
 {
-    int rc = hvm_msr_write_intercept(reg, val, true);
+    int rc = hvm_msr_write_intercept(reg, val, explicit);
 
     if ( rc == X86EMUL_EXCEPTION )
         x86_emul_hw_exception(X86_EXC_GP, 0, ctxt);
diff --git a/xen/arch/x86/pv/emul-priv-op.c b/xen/arch/x86/pv/emul-priv-op.c
index c970e16152..1d9e57ad27 100644
--- a/xen/arch/x86/pv/emul-priv-op.c
+++ b/xen/arch/x86/pv/emul-priv-op.c
@@ -1038,7 +1038,8 @@ static int cf_check read_msr(
 }
 
 static int cf_check write_msr(
-    unsigned int reg, uint64_t val, struct x86_emulate_ctxt *ctxt)
+    unsigned int reg, uint64_t val, struct x86_emulate_ctxt *ctxt,
+    bool explicit)
 {
     struct vcpu *curr = current;
     const struct domain *currd = curr->domain;
diff --git a/xen/arch/x86/x86_emulate/0f01.c b/xen/arch/x86/x86_emulate/0f01.c
index 1ba99609d6..6c10979dd6 100644
--- a/xen/arch/x86/x86_emulate/0f01.c
+++ b/xen/arch/x86/x86_emulate/0f01.c
@@ -40,7 +40,7 @@ int x86emul_0f01(struct x86_emulate_state *s,
             fail_if(!ops->write_msr);
             rc = ops->write_msr(regs->ecx,
                                 ((uint64_t)regs->r(dx) << 32) | regs->eax,
-                                ctxt);
+                                ctxt, true);
             goto done;
         }
         generate_exception(X86_EXC_UD);
@@ -194,7 +194,7 @@ int x86emul_0f01(struct x86_emulate_state *s,
              (rc = ops->read_msr(MSR_SHADOW_GS_BASE, &msr_val,
                                  ctxt)) != X86EMUL_OKAY ||
              (rc = ops->write_msr(MSR_SHADOW_GS_BASE, sreg.base,
-                                  ctxt)) != X86EMUL_OKAY )
+                                  ctxt, false)) != X86EMUL_OKAY )
             goto done;
         sreg.base = msr_val;
         if ( (rc = ops->write_segment(x86_seg_gs, &sreg,
@@ -202,7 +202,7 @@ int x86emul_0f01(struct x86_emulate_state *s,
         {
             /* Best effort unwind (i.e. no real error checking). */
             if ( ops->write_msr(MSR_SHADOW_GS_BASE, msr_val,
-                                ctxt) == X86EMUL_EXCEPTION )
+                                ctxt, false) == X86EMUL_EXCEPTION )
                 x86_emul_reset_event(ctxt);
             goto done;
         }
diff --git a/xen/arch/x86/x86_emulate/x86_emulate.c 
b/xen/arch/x86/x86_emulate/x86_emulate.c
index d830aea430..7751a67130 100644
--- a/xen/arch/x86/x86_emulate/x86_emulate.c
+++ b/xen/arch/x86/x86_emulate/x86_emulate.c
@@ -3814,7 +3814,7 @@ x86_emulate(
         fail_if(ops->write_msr == NULL);
         if ( (rc = ops->write_msr(_regs.ecx,
                                   ((uint64_t)_regs.r(dx) << 32) | _regs.eax,
-                                  ctxt)) != 0 )
+                                  ctxt, true)) != 0 )
             goto done;
         break;
 
diff --git a/xen/arch/x86/x86_emulate/x86_emulate.h 
b/xen/arch/x86/x86_emulate/x86_emulate.h
index d75658eba0..0fd20747dc 100644
--- a/xen/arch/x86/x86_emulate/x86_emulate.h
+++ b/xen/arch/x86/x86_emulate/x86_emulate.h
@@ -470,13 +470,15 @@ struct x86_emulate_ops
         struct x86_emulate_ctxt *ctxt);
 
     /*
-     * write_dr: Write to model-specific register.
-     *  @reg:   [IN ] Register to write.
+     * write_msr: Write to model-specific register.
+     *  @reg:      [IN ] Register to write.
+     *  @explicit: [IN ] Whether this is an explicit WRMSR or alike.
      */
     int (*write_msr)(
         unsigned int reg,
         uint64_t val,
-        struct x86_emulate_ctxt *ctxt);
+        struct x86_emulate_ctxt *ctxt,
+        bool explicit);
 
     /*
      * cache_op: Write-back and/or invalidate cache contents.
--
generated by git-patchbot for /home/xen/git/xen.git#staging