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Guest-side KVM async #PF ABI cleanup for 6.9 

Delete kvm_vcpu_pv_apf_data.enabled to fix a goof in KVM's async #PF ABI where
 the enabled field pushes the size of "struct kvm_vcpu_pv_apf_data" from 64 to
 68 bytes, i.e. beyond a single cache line.
 
 The enabled field is purely a guest-side flag that Linux-as-a-guest uses to
 track whether or not the guest has enabled async #PF support.  The actual flag
 that is passed to the host, i.e. to KVM proper, is a single bit in a synthetic
 MSR, MSR_KVM_ASYNC_PF_EN, i.e. is in a location completely unrelated to the
 shared kvm_vcpu_pv_apf_data structure.
 
 Simply drop the the field and use a dedicated guest-side per-CPU variable to
 fix the ABI, as opposed to fixing the documentation to match reality.  KVM has
 never consumed kvm_vcpu_pv_apf_data.enabled, so the odds of the ABI change
 breaking anything are extremely low.
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Merge tag 'kvm-x86-asyncpf_abi-6.9' of https://github.com/kvm-x86/linux into HEAD

Guest-side KVM async #PF ABI cleanup for 6.9

Delete kvm_vcpu_pv_apf_data.enabled to fix a goof in KVM's async #PF ABI where
the enabled field pushes the size of "struct kvm_vcpu_pv_apf_data" from 64 to
68 bytes, i.e. beyond a single cache line.

The enabled field is purely a guest-side flag that Linux-as-a-guest uses to
track whether or not the guest has enabled async #PF support.  The actual flag
that is passed to the host, i.e. to KVM proper, is a single bit in a synthetic
MSR, MSR_KVM_ASYNC_PF_EN, i.e. is in a location completely unrelated to the
shared kvm_vcpu_pv_apf_data structure.

Simply drop the the field and use a dedicated guest-side per-CPU variable to
fix the ABI, as opposed to fixing the documentation to match reality.  KVM has
never consumed kvm_vcpu_pv_apf_data.enabled, so the odds of the ABI change
breaking anything are extremely low.
This commit is contained in:
Paolo Bonzini 2024-03-18 19:03:42 -04:00
parent 1d55934ed5 df01f0a116
commit c822a075ab
3 changed files with 15 additions and 16 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

19
Documentation/virt/kvm/x86/msr.rst
View File

@ -193,8 +193,8 @@ data:
Asynchronous page fault (APF) control MSR.
Bits 63-6 hold 64-byte aligned physical address of a 64 byte memory area
which must be in guest RAM and must be zeroed. This memory is expected
to hold a copy of the following structure::
which must be in guest RAM. This memory is expected to hold the
following structure::
struct kvm_vcpu_pv_apf_data {
/* Used for 'page not present' events delivered via #PF */
@ -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
@ -232,14 +231,14 @@ data:
as regular page fault, guest must reset 'flags' to '0' before it does
something that can generate normal page fault.
Bytes 5-7 of 64 byte memory location ('token') will be written to by the
Bytes 4-7 of 64 byte memory location ('token') will be written to by the
hypervisor at the time of APF 'page ready' event injection. The content
of these bytes is a token which was previously delivered as 'page not
present' event. The event indicates the page in now available. Guest is
supposed to write '0' to 'token' when it is done handling 'page ready'
event and to write 1' to MSR_KVM_ASYNC_PF_ACK after clearing the location;
writing to the MSR forces KVM to re-scan its queue and deliver the next
pending notification.
of these bytes is a token which was previously delivered in CR2 as
'page not present' event. The event indicates the page is now available.
Guest is supposed to write '0' to 'token' when it is done handling
'page ready' event and to write '1' to MSR_KVM_ASYNC_PF_ACK after
clearing the location; writing to the MSR forces KVM to re-scan its
queue and deliver the next pending notification.
Note, MSR_KVM_ASYNC_PF_INT MSR specifying the interrupt vector for 'page
ready' APF delivery needs to be written to before enabling APF mechanism

1
arch/x86/include/uapi/asm/kvm_para.h
View File

@ -142,7 +142,6 @@ struct kvm_vcpu_pv_apf_data {
__u32 token;
__u8 pad[56];
__u32 enabled;
};
#define KVM_PV_EOI_BIT 0

11
arch/x86/kernel/kvm.c
View File

@ -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);
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, 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, false);
pr_debug("disable async PF for cpu %d\n", smp_processor_id());
}