[PATCH qemu v5 1/1] Honor guest PAT on x86, absent Bochs display

[PATCH qemu v5 1/1] Honor guest PAT on x86, absent Bochs display

[~myrslint]

From: myrslint <qemu.haziness801@passinbox.com>

On x86_64, where most CPUs support self-snoop, it is preferrable to
always honor guest PAT. Not doing so is a quirk. There is a default
enabled KVM quirk flag which enforces not doing so due to a former bug
in Bochs display driver.

The bug has been fixed but not enough has yet passed since so we only
disable said quirk flag if a Bochs display is not configured for the
virtual machine.

This commit also moves around a bit of code that would be called when
the initialization of a VM is done.
---
 hw/display/bochs-display.c |  4 +++
 include/system/kvm.h       |  1 +
 target/i386/kvm/kvm.c      | 52 ++++++++++++++++++++++++++++++++------
 3 files changed, 49 insertions(+), 8 deletions(-)

diff --git a/hw/display/bochs-display.c b/hw/display/bochs-display.c
index ad2821c974..4059373df3 100644
--- a/hw/display/bochs-display.c
+++ b/hw/display/bochs-display.c
@@ -20,6 +20,8 @@
 #include "ui/qemu-pixman.h"
 #include "qom/object.h"
 
+#include "system/kvm.h"
+
 typedef struct BochsDisplayMode {
     pixman_format_code_t format;
     uint32_t             bytepp;
@@ -309,6 +311,8 @@ static void bochs_display_realize(PCIDevice *dev, Error **errp)
     }
 
     memory_region_set_log(&s->vram, true, DIRTY_MEMORY_VGA);
+
+    kvm_hack_do_not_enable_guest_pat();
 }
 
 static bool bochs_display_get_big_endian_fb(Object *obj, Error **errp)
diff --git a/include/system/kvm.h b/include/system/kvm.h
index 3c7d314736..c6ba3905dc 100644
--- a/include/system/kvm.h
+++ b/include/system/kvm.h
@@ -208,6 +208,7 @@ void kvm_close(void);
  */
 bool kvm_arm_supports_user_irq(void);
 
+void kvm_hack_do_not_enable_guest_pat(void);
 
 int kvm_on_sigbus_vcpu(CPUState *cpu, int code, void *addr);
 int kvm_on_sigbus(int code, void *addr);
diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
index 369626f8c8..0f8af92962 100644
--- a/target/i386/kvm/kvm.c
+++ b/target/i386/kvm/kvm.c
@@ -188,6 +188,8 @@ static KVMMSRHandlers msr_handlers[KVM_MSR_FILTER_MAX_RANGES];
 static RateLimit bus_lock_ratelimit_ctrl;
 static int kvm_get_one_msr(X86CPU *cpu, int index, uint64_t *value);
 
+bool kvm_has_bochs_drm = false;
+
 static const char *vm_type_name[] = {
     [KVM_X86_DEFAULT_VM] = "default",
     [KVM_X86_SEV_VM] = "SEV",
@@ -2696,13 +2698,13 @@ static bool kvm_rdmsr_pkg_energy_status(X86CPU *cpu,
     return true;
 }
 
-static Notifier smram_machine_done;
+static Notifier kvm_machine_done;
 static KVMMemoryListener smram_listener;
 static AddressSpace smram_address_space;
 static MemoryRegion smram_as_root;
 static MemoryRegion smram_as_mem;
 
-static void register_smram_listener(Notifier *n, void *unused)
+static void register_smram_listener(void)
 {
     MemoryRegion *smram =
         (MemoryRegion *) object_resolve_path("/machine/smram", NULL);
@@ -2730,6 +2732,44 @@ static void register_smram_listener(Notifier *n, void *unused)
                                  &smram_address_space, 1, "kvm-smram");
 }
 
+void kvm_hack_do_not_enable_guest_pat(void)
+{
+    kvm_has_bochs_drm = true;
+}
+
+static bool kvm_x86_smm_enabled(void)
+{
+    return kvm_has_smm() &&
+        object_dynamic_cast(OBJECT(current_machine), TYPE_X86_MACHINE) &&
+        x86_machine_is_smm_enabled(X86_MACHINE(current_machine));
+}
+
+static int kvm_x86_disable_quirsk2_mask(void)
+{
+    return kvm_check_extension(kvm_state, KVM_CAP_DISABLE_QUIRKS2);
+}
+
+static int kvm_disable_ignore_guest_pat(void)
+{
+    return kvm_vm_enable_cap(kvm_state, KVM_CAP_DISABLE_QUIRKS2, 0, \
+                             KVM_X86_QUIRK_IGNORE_GUEST_PAT);
+}
+
+static void handle_machine_done(Notifier *n, void *unused)
+{
+    if (kvm_x86_smm_enabled()) {
+        register_smram_listener();
+    }
+    if (!kvm_has_bochs_drm && \
+        (kvm_x86_disable_quirsk2_mask() & KVM_X86_QUIRK_IGNORE_GUEST_PAT)) {
+        if (kvm_disable_ignore_guest_pat()) {
+            error_report("KVM_X86_QUIRK_IGNORE_GUEST_PAT available and "
+                         "modifiable but we failed to disable it\n");
+        }
+    }
+}
+
+
 static void *kvm_msr_energy_thread(void *data)
 {
     KVMState *s = data;
@@ -3310,12 +3350,8 @@ int kvm_arch_init(MachineState *ms, KVMState *s)
         return ret;
     }
 
-    if (kvm_check_extension(s, KVM_CAP_X86_SMM) &&
-        object_dynamic_cast(OBJECT(ms), TYPE_X86_MACHINE) &&
-        x86_machine_is_smm_enabled(X86_MACHINE(ms))) {
-        smram_machine_done.notify = register_smram_listener;
-        qemu_add_machine_init_done_notifier(&smram_machine_done);
-    }
+    kvm_machine_done.notify = handle_machine_done;
+    qemu_add_machine_init_done_notifier(&kvm_machine_done);
 
     if (enable_cpu_pm) {
         ret = kvm_vm_enable_disable_exits(s);
-- 
2.49.1

[PATCH qemu v5 0/1] Honor guest PAT on x86, absent Bochs display

[~myrslint]

This is an attempt to follow Paolo Bonzini's <pbonzini@redhat.com>
guidance on my previous proposed patch (Default disable ignore guest PAT
quirk). Hopefully, it correctly applies the advice.

myrslint (1):
  Honor guest PAT on x86, absent Bochs display

 hw/display/bochs-display.c |  4 +++
 include/system/kvm.h       |  1 +
 target/i386/kvm/kvm.c      | 52 ++++++++++++++++++++++++++++++++------
 3 files changed, 49 insertions(+), 8 deletions(-)

-- 
2.49.1

Re: [PATCH qemu v5 1/1] Honor guest PAT on x86, absent Bochs display

[Dmitry Osipenko]

Hi,

On 8/17/25 08:37, ~myrslint wrote:
> From: myrslint <qemu.haziness801@passinbox.com>
> 
> On x86_64, where most CPUs support self-snoop, it is preferrable to
> always honor guest PAT. Not doing so is a quirk. There is a default
> enabled KVM quirk flag which enforces not doing so due to a former bug
> in Bochs display driver.
> 
> The bug has been fixed but not enough has yet passed since so we only
> disable said quirk flag if a Bochs display is not configured for the
> virtual machine.
> 
> This commit also moves around a bit of code that would be called when
> the initialization of a VM is done.
> ---
>  hw/display/bochs-display.c |  4 +++
>  include/system/kvm.h       |  1 +
>  target/i386/kvm/kvm.c      | 52 ++++++++++++++++++++++++++++++++------
>  3 files changed, 49 insertions(+), 8 deletions(-)
...

> diff --git a/include/system/kvm.h b/include/system/kvm.h
> index 3c7d314736..c6ba3905dc 100644
> --- a/include/system/kvm.h
> +++ b/include/system/kvm.h
> @@ -208,6 +208,7 @@ void kvm_close(void);
>   */
>  bool kvm_arm_supports_user_irq(void);
>  
> +void kvm_hack_do_not_enable_guest_pat(void);

This will fail to compile if KVM is disabled or if target != i386.
AFAICT, you'll need to check whether KVM is available at build time and
move common code to kvm-all.c.

#if defined(CONFIG_KVM_IS_POSSIBLE)
void kvm_hack_do_not_enable_guest_pat(void);
#else
static inline void kvm_hack_do_not_enable_guest_pat(void)
{
}
#endif

-- 
Best regards,
Dmitry