From ccb2280ec2f9e805d70f57a3a1c5deff0d532cb3 Mon Sep 17 00:00:00 2001 From: Xiaoyao Li Date: Wed, 25 Oct 2023 01:59:13 -0400 Subject: x86/kvm: Use separate percpu variable to track the enabling of asyncpf 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. 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@google.com/T/#u Suggested-by: Sean Christopherson Signed-off-by: Xiaoyao Li Link: https://lore.kernel.org/r/20231025055914.1201792-2-xiaoyao.li@intel.com [sean: use true/false instead of 1/0 for booleans] Signed-off-by: Sean Christopherson --- Documentation/virt/kvm/x86/msr.rst | 1 - 1 file changed, 1 deletion(-) (limited to 'Documentation/virt') 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 -- cgit v1.2.3 From df01f0a1165c35e95b5f52c7ba25c19020352ff9 Mon Sep 17 00:00:00 2001 From: Xiaoyao Li Date: Wed, 25 Oct 2023 01:59:14 -0400 Subject: KVM: x86: Improve documentation of MSR_KVM_ASYNC_PF_EN Fix some incorrect statement of MSR_KVM_ASYNC_PF_EN documentation and state clearly the token in 'struct kvm_vcpu_pv_apf_data' of 'page ready' event is matchted with the token in CR2 in 'page not present' event. Signed-off-by: Xiaoyao Li Link: https://lore.kernel.org/r/20231025055914.1201792-3-xiaoyao.li@intel.com Signed-off-by: Sean Christopherson --- Documentation/virt/kvm/x86/msr.rst | 18 +++++++++--------- 1 file changed, 9 insertions(+), 9 deletions(-) (limited to 'Documentation/virt') diff --git a/Documentation/virt/kvm/x86/msr.rst b/Documentation/virt/kvm/x86/msr.rst index f6d70f99a1a7..3aecf2a70e7b 100644 --- a/Documentation/virt/kvm/x86/msr.rst +++ b/Documentation/virt/kvm/x86/msr.rst @@ -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 */ @@ -231,14 +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 -- cgit v1.2.3