Xiaoyao Li <xiaoyao.li@xxxxxxxxx> writes:
Refer to commit fd10cde9294f ("KVM paravirt: Add async PF initialization
to PV guest") and commit 344d9588a9df ("KVM: Add PV MSR to enable
asynchronous page faults delivery"). It turns out that at the time when
asyncpf was introduced, the purpose was defining the shared PV data 'struct
kvm_vcpu_pv_apf_data' with the size of 64 bytes. However, it made a mistake
and defined the size to 68 bytes, which failed to make fit in a cache line
and made the code inconsistent with the documentation.
Oh, I actually though it was done on purpose :-) 'enabled' is not
accessed by the host, it's only purpose is to cache MSR_KVM_ASYNC_PF_EN.
Below justification quoted from Sean[*]
KVM (the host side) has *never* read kvm_vcpu_pv_apf_data.enabled, and
the documentation clearly states that enabling is based solely on the
bit in the synthetic MSR.
So rather than update the documentation, fix the goof by removing the
enabled filed and use the separate percpu variable instread.
KVM-as-a-host obviously doesn't enforce anything or consume the size,
and changing the header will only affect guests that are rebuilt against
the new header, so there's no chance of ABI breakage between KVM and its
guests. The only possible breakage is if some other hypervisor is
emulating KVM's async #PF (LOL) and relies on the guest to set
kvm_vcpu_pv_apf_data.enabled. But (a) I highly doubt such a hypervisor
exists, (b) that would arguably be a violation of KVM's "spec", and
(c) the worst case scenario is that the guest would simply lose async
#PF functionality.
[*] https://lore.kernel.org/all/ZS7ERnnRqs8Fl0ZF@xxxxxxxxxx/T/#u
Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Signed-off-by: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>
---
Documentation/virt/kvm/x86/msr.rst | 1 -
arch/x86/include/uapi/asm/kvm_para.h | 1 -
arch/x86/kernel/kvm.c | 11 ++++++-----
3 files changed, 6 insertions(+), 7 deletions(-)
diff --git a/Documentation/virt/kvm/x86/msr.rst b/Documentation/virt/kvm/x86/msr.rst
index 9315fc385fb0..f6d70f99a1a7 100644
--- a/Documentation/virt/kvm/x86/msr.rst
+++ b/Documentation/virt/kvm/x86/msr.rst
@@ -204,7 +204,6 @@ data:
__u32 token;
__u8 pad[56];
- __u32 enabled;
};
Bits 5-4 of the MSR are reserved and should be zero. Bit 0 is set to 1
diff --git a/arch/x86/include/uapi/asm/kvm_para.h b/arch/x86/include/uapi/asm/kvm_para.h
index 6e64b27b2c1e..605899594ebb 100644
--- a/arch/x86/include/uapi/asm/kvm_para.h
+++ b/arch/x86/include/uapi/asm/kvm_para.h
@@ -142,7 +142,6 @@ struct kvm_vcpu_pv_apf_data {
__u32 token;
__u8 pad[56];
- __u32 enabled;
};
#define KVM_PV_EOI_BIT 0
diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index b8ab9ee5896c..388a3fdd3cad 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -65,6 +65,7 @@ static int __init parse_no_stealacc(char *arg)
early_param("no-steal-acc", parse_no_stealacc);
+static DEFINE_PER_CPU_READ_MOSTLY(bool, async_pf_enabled);
Would it make a difference is we replace this with a cpumask? I realize
that we need to access it on all CPUs from hotpaths but this mask will
rarely change so maybe there's no real perfomance hit?
static DEFINE_PER_CPU_DECRYPTED(struct kvm_vcpu_pv_apf_data, apf_reason) __aligned(64);
DEFINE_PER_CPU_DECRYPTED(struct kvm_steal_time, steal_time) __aligned(64) __visible;
static int has_steal_clock = 0;
@@ -244,7 +245,7 @@ noinstr u32 kvm_read_and_reset_apf_flags(void)
{
u32 flags = 0;
- if (__this_cpu_read(apf_reason.enabled)) {
+ if (__this_cpu_read(async_pf_enabled)) {
flags = __this_cpu_read(apf_reason.flags);
__this_cpu_write(apf_reason.flags, 0);
}
@@ -295,7 +296,7 @@ DEFINE_IDTENTRY_SYSVEC(sysvec_kvm_asyncpf_interrupt)
inc_irq_stat(irq_hv_callback_count);
- if (__this_cpu_read(apf_reason.enabled)) {
+ if (__this_cpu_read(async_pf_enabled)) {
token = __this_cpu_read(apf_reason.token);
kvm_async_pf_task_wake(token);
__this_cpu_write(apf_reason.token, 0);
@@ -362,7 +363,7 @@ static void kvm_guest_cpu_init(void)
wrmsrl(MSR_KVM_ASYNC_PF_INT, HYPERVISOR_CALLBACK_VECTOR);
wrmsrl(MSR_KVM_ASYNC_PF_EN, pa);
- __this_cpu_write(apf_reason.enabled, 1);
+ __this_cpu_write(async_pf_enabled, 1);
As 'async_pf_enabled' is bool, it would probably be more natural to
write
__this_cpu_write(async_pf_enabled, true);
pr_debug("setup async PF for cpu %d\n", smp_processor_id());
}
@@ -383,11 +384,11 @@ static void kvm_guest_cpu_init(void)
static void kvm_pv_disable_apf(void)
{
- if (!__this_cpu_read(apf_reason.enabled))
+ if (!__this_cpu_read(async_pf_enabled))
return;
wrmsrl(MSR_KVM_ASYNC_PF_EN, 0);
- __this_cpu_write(apf_reason.enabled, 0);
+ __this_cpu_write(async_pf_enabled, 0);
... and 'false' here.
