[xen stable-4.19] x86/vlapic: Fix handling of writes to APIC_ESR

[patchbot@xxxxxxx]

commit da239140a48eb9b2b30784ee5dd420b6f879d189
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Thu Mar 20 13:18:23 2025 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Thu Mar 20 13:18:23 2025 +0100

    x86/vlapic: Fix handling of writes to APIC_ESR
    
    Xen currently presents APIC_ESR to guests as a simple read/write register.
    
    This is incorrect.  The SDM states:
    
      The ESR is a write/read register. Before attempt to read from the ESR,
      software should first write to it. (The value written does not affect the
      values read subsequently; only zero may be written in x2APIC mode.) This
      write clears any previously logged errors and updates the ESR with any
      errors detected since the last write to the ESR.
    
    Introduce a new pending_esr field in hvm_hw_lapic.
    
    Update vlapic_error() to accumulate errors here, and extend 
vlapic_reg_write()
    to discard the written value and transfer pending_esr into APIC_ESR.  Reads
    are still as before.
    
    Importantly, this means that guests no longer destroys the ESR value it's
    looking for in the LVTERR handler when following the SDM instructions.
    
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
    master commit: b28b590d4a23894672f1dd7fb98cdf9926ecb282
    master date: 2025-03-07 14:34:08 +0000
---
 xen/arch/x86/hvm/vlapic.c              | 17 +++++++++++++++--
 xen/include/public/arch-x86/hvm/save.h |  1 +
 2 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/hvm/vlapic.c b/xen/arch/x86/hvm/vlapic.c
index 46ff758904..f34bbfb3cc 100644
--- a/xen/arch/x86/hvm/vlapic.c
+++ b/xen/arch/x86/hvm/vlapic.c
@@ -108,7 +108,7 @@ static void vlapic_error(struct vlapic *vlapic, unsigned 
int errmask)
     uint32_t esr;
 
     spin_lock_irqsave(&vlapic->esr_lock, flags);
-    esr = vlapic_get_reg(vlapic, APIC_ESR);
+    esr = vlapic->hw.pending_esr;
     if ( (esr & errmask) != errmask )
     {
         uint32_t lvterr = vlapic_get_reg(vlapic, APIC_LVTERR);
@@ -127,7 +127,7 @@ static void vlapic_error(struct vlapic *vlapic, unsigned 
int errmask)
                  errmask |= APIC_ESR_RECVILL;
         }
 
-        vlapic_set_reg(vlapic, APIC_ESR, esr | errmask);
+        vlapic->hw.pending_esr |= errmask;
 
         if ( inj )
             vlapic_set_irq(vlapic, lvterr & APIC_VECTOR_MASK, 0);
@@ -799,6 +799,19 @@ void vlapic_reg_write(struct vcpu *v, unsigned int reg, 
uint32_t val)
         vlapic_set_reg(vlapic, APIC_ID, val);
         break;
 
+    case APIC_ESR:
+    {
+        unsigned long flags;
+
+        spin_lock_irqsave(&vlapic->esr_lock, flags);
+        val = vlapic->hw.pending_esr;
+        vlapic->hw.pending_esr = 0;
+        spin_unlock_irqrestore(&vlapic->esr_lock, flags);
+
+        vlapic_set_reg(vlapic, APIC_ESR, val);
+        break;
+    }
+
     case APIC_TASKPRI:
         vlapic_set_reg(vlapic, APIC_TASKPRI, val & 0xff);
         break;
diff --git a/xen/include/public/arch-x86/hvm/save.h 
b/xen/include/public/arch-x86/hvm/save.h
index 7ecacadde1..9c4bfc7ebd 100644
--- a/xen/include/public/arch-x86/hvm/save.h
+++ b/xen/include/public/arch-x86/hvm/save.h
@@ -394,6 +394,7 @@ struct hvm_hw_lapic {
     uint32_t             disabled; /* VLAPIC_xx_DISABLED */
     uint32_t             timer_divisor;
     uint64_t             tdt_msr;
+    uint32_t             pending_esr;
 };
 
 DECLARE_HVM_SAVE_TYPE(LAPIC, 5, struct hvm_hw_lapic);
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.19