[PATCH] x86/vhpet: Fix sanitization of legacy IRQ route

["Tu Dinh" <ngoc-tu.dinh@xxxxxxxxxx>]

When setting a timer's config register, timer_sanitize_int_route will
always reset the IRQ route value to what's valid corresponding to the
!HPET_CFG_LEGACY case. This is applied even if the HPET is set to
HPET_CFG_LEGACY.

When some operating systems (e.g. Windows) try to write to a timer
config, they will verify and rewrite the register if the values don't
match what they expect. This causes an unnecessary write to HPET_Tn_CFG.

Note, the HPET specification states that for the Tn_INT_ROUTE_CNF field:

"If the value is not supported by this prarticular timer, then the value
read back will not match what is written. [...] If the LegacyReplacement
Route bit is set, then Timers 0 and 1 will have a different routing, and
this bit field has no effect for those two timers."

Therefore, Xen should not reset timer_int_route if legacy mode is
enabled, regardless of what's in there.

Signed-off-by: Tu Dinh <ngoc-tu.dinh@xxxxxxxxxx>
---
 xen/arch/x86/hvm/hpet.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/xen/arch/x86/hvm/hpet.c b/xen/arch/x86/hvm/hpet.c
index f0e5f877f4..fb2f4f94aa 100644
--- a/xen/arch/x86/hvm/hpet.c
+++ b/xen/arch/x86/hvm/hpet.c
@@ -48,6 +48,8 @@
 #define timer_is_32bit(h, n)     (timer_config(h, n) & HPET_TN_32BIT)
 #define hpet_enabled(h)          ((h)->hpet.config & HPET_CFG_ENABLE)
 #define timer_level(h, n)        (timer_config(h, n) & HPET_TN_LEVEL)
+#define timer_is_legacy(h, n) \
+    (((n) <= 1) && ((h)->hpet.config & HPET_CFG_LEGACY))
 
 #define timer_int_route(h, n)    MASK_EXTR(timer_config(h, n), HPET_TN_ROUTE)
 
@@ -244,7 +246,7 @@ static void hpet_set_timer(HPETState *h, unsigned int tn,
          (timer_level(h, tn) && test_bit(tn, &h->hpet.isr)) )
         return;
 
-    if ( !timer_int_route_valid(h, tn) )
+    if ( !timer_is_legacy(h, tn) && !timer_int_route_valid(h, tn) )
     {
         ASSERT_UNREACHABLE();
         return;
@@ -275,7 +277,7 @@ static void hpet_set_timer(HPETState *h, unsigned int tn,
             ? (uint32_t)diff : 0;
 
     destroy_periodic_time(&h->pt[tn]);
-    if ( (tn <= 1) && (h->hpet.config & HPET_CFG_LEGACY) )
+    if ( timer_is_legacy(h, tn) )
     {
         /* if LegacyReplacementRoute bit is set, HPET specification requires
            timer0 be routed to IRQ0 in NON-APIC or IRQ2 in the I/O APIC,
@@ -323,7 +325,7 @@ static inline uint64_t hpet_fixup_reg(
 
 static void timer_sanitize_int_route(HPETState *h, unsigned int tn)
 {
-    if ( timer_int_route_valid(h, tn) )
+    if ( timer_is_legacy(h, tn) || timer_int_route_valid(h, tn) )
         return;
 
     timer_config(h, tn) &= ~HPET_TN_ROUTE;
@@ -379,6 +381,9 @@ static int cf_check hpet_write(
         h->hpet.config = hpet_fixup_reg(new_val, old_val,
                                         HPET_CFG_ENABLE | HPET_CFG_LEGACY);
 
+        for ( i = 0; i < HPET_TIMER_NUM; i++ )
+            timer_sanitize_int_route(h, i);
+
         if ( !(old_val & HPET_CFG_ENABLE) && (new_val & HPET_CFG_ENABLE) )
         {
             /* Enable main counter and interrupt generation. */
-- 
2.43.0



--
Ngoc Tu Dinh | Vates XCP-ng Developer

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