* [PATCH qemu v8 1/1] Honor guest PAT on x86, absent Bochs display
2025-10-14 16:32 [PATCH qemu v8 0/1] Honor guest PAT on x86, absent Bochs display ~myrslint
@ 2025-10-14 16:23 ` ~myrslint
2025-10-14 20:01 ` Dmitry Osipenko
0 siblings, 1 reply; 4+ messages in thread
From: ~myrslint @ 2025-10-14 16:23 UTC (permalink / raw)
To: qemu-devel; +Cc: Dmitry Osipenko, Paolo Bonzini, Gerd Hoffmann
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.
Signed-off-by: myrslint <qemu.haziness801@passinbox.com>
---
accel/kvm/kvm-all.c | 1 +
accel/stubs/kvm-stub.c | 1 +
hw/display/bochs-display.c | 17 +++++++++++++++
include/system/kvm.h | 9 ++++++++
target/i386/kvm/kvm.c | 42 +++++++++++++++++++++++++++++++-------
5 files changed, 63 insertions(+), 7 deletions(-)
diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
index 58802f7c3c..b1ae36d6a1 100644
--- a/accel/kvm/kvm-all.c
+++ b/accel/kvm/kvm-all.c
@@ -103,6 +103,7 @@ bool kvm_readonly_mem_allowed;
bool kvm_vm_attributes_allowed;
bool kvm_msi_use_devid;
bool kvm_pre_fault_memory_supported;
+bool kvm_bochs_drm_quirk;
static bool kvm_has_guest_debug;
static int kvm_sstep_flags;
static bool kvm_immediate_exit;
diff --git a/accel/stubs/kvm-stub.c b/accel/stubs/kvm-stub.c
index 68cd33ba97..a69c89e1ad 100644
--- a/accel/stubs/kvm-stub.c
+++ b/accel/stubs/kvm-stub.c
@@ -24,6 +24,7 @@ bool kvm_gsi_direct_mapping;
bool kvm_allowed;
bool kvm_readonly_mem_allowed;
bool kvm_msi_use_devid;
+bool kvm_bochs_drm_quirk;
void kvm_flush_coalesced_mmio_buffer(void)
{
diff --git a/hw/display/bochs-display.c b/hw/display/bochs-display.c
index ad2821c974..e8def68b41 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 +311,21 @@ static void bochs_display_realize(PCIDevice *dev, Error **errp)
}
memory_region_set_log(&s->vram, true, DIRTY_MEMORY_VGA);
+
+ /*
+ * 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.
+ *
+ * The following flag tells KVM initialization code not to disable that
+ * quirk flag.
+ */
+ kvm_bochs_drm_quirk = true;
}
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 4fc09e3891..45ddb2e0ee 100644
--- a/include/system/kvm.h
+++ b/include/system/kvm.h
@@ -43,6 +43,7 @@ extern bool kvm_gsi_direct_mapping;
extern bool kvm_readonly_mem_allowed;
extern bool kvm_msi_use_devid;
extern bool kvm_pre_fault_memory_supported;
+extern bool kvm_bochs_drm_quirk;
#define kvm_enabled() (kvm_allowed)
/**
@@ -144,6 +145,13 @@ extern bool kvm_pre_fault_memory_supported;
*/
#define kvm_msi_devid_required() (kvm_msi_use_devid)
+/**
+ * kvm_has_bochs_drm:
+ * Returns: true if KVM is possible and a Bochs DRM driver is
+ * in use for display.
+ */
+#define kvm_has_bochs_drm() (kvm_bochs_drm_quirk)
+
#else
#define kvm_enabled() (0)
@@ -158,6 +166,7 @@ extern bool kvm_pre_fault_memory_supported;
#define kvm_gsi_direct_mapping() (false)
#define kvm_readonly_mem_enabled() (false)
#define kvm_msi_devid_required() (false)
+#define kvm_has_bochs_drm() (false)
#endif /* CONFIG_KVM_IS_POSSIBLE */
diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
index db40caa341..4d2a01ca0e 100644
--- a/target/i386/kvm/kvm.c
+++ b/target/i386/kvm/kvm.c
@@ -2692,13 +2692,13 @@ 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)
{
CPUState *cpu;
MemoryRegion *smram =
@@ -2731,6 +2731,37 @@ static void register_smram_listener(Notifier *n, void *unused)
}
}
+static bool kvm_x86_smm_enabled(void)
+{
+ return 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");
+ }
+ }
+}
+
static void *kvm_msr_energy_thread(void *data)
{
KVMState *s = data;
@@ -3311,11 +3342,8 @@ int kvm_arch_init(MachineState *ms, KVMState *s)
return ret;
}
- if (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
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