Messages in this thread | | | Subject | Re: [PATCH 1/2] kvm: vmx: Use basic exit reason to check if it's the specific VM EXIT | From | Xiaoyao Li <> | Date | Tue, 25 Feb 2020 14:41:20 +0800 |
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On 2/25/2020 2:13 PM, Sean Christopherson wrote: > On Tue, Feb 25, 2020 at 08:13:15AM +0800, Xiaoyao Li wrote: >> On 2/25/2020 12:17 AM, Sean Christopherson wrote: >>> On Mon, Feb 24, 2020 at 02:04:46PM +0100, Vitaly Kuznetsov wrote: >>>> Xiaoyao Li <xiaoyao.li@intel.com> writes: >>>> >>>>> On 2/24/2020 6:16 PM, Vitaly Kuznetsov wrote: >>>>>> Xiaoyao Li <xiaoyao.li@intel.com> writes: >>>>>> >>>>> Here variable basic_exit_reason is added for the cases where only basic >>>>> exit reason number is needed. >>>>> >>>> >>>> Can we do the other way around, i.e. introduce 'extended_exit_reason' >>>> and use it where all 32 bits are needed? I'm fine with the change, just >>>> trying to minimize the (unneeded) code churn. >>> >>> 100% agree. Even better than adding a second field to vcpu_vmx would be >>> to make it a union, though we'd probably want to call it something like >>> full_exit_reason in that case. That should give us compile-time checks on >>> exit_reason, e.g. if we try to query one of the upper bits using a u16, e.g. >> >> I have thought about union, but it seems >> >> union { >> u16 exit_reason; >> u32 full_exit_reason; >> } >> >> is not a good name. Since there are many codes in vmx.c and nested.c assume >> that exit_reason stands for 32-bit EXIT REASON vmcs field as well as >> evmcs->vm_exit_reason and vmcs12->vm_exit_reason. Do we really want to also >> rename them to full_exit_reason? > > It's actually the opposite, almost all of the VMX code assumes exit_reason > holds only the basic exit reason, i.e. a 16-bit value. For example, SGX > adds a modifier flag to denote a VM-Exit was from enclave mode, and that > bit needs to be stripped from exit_reason, otherwise all the checks like > "if (exit_reason == blah_blah_blah)" fail. > > Making exit_reason a 16-bit alias of the full/extended exit_reason neatly > sidesteps that issue. And it is an issue that has caused actual problems > in the past, e.g. see commit beb8d93b3e42 ("KVM: VMX: Fix handling of #MC > that occurs during VM-Entry"). Coincidentally, that commit also removes a > local "u16 basic_exit_reason" :-). > > Except for one mistake, the pseudo-patch below is the entirety of required > changes. Most (all?) of the functions that take "u32 exit_reason" can (and > should) continue to take a u32. > > As for the name, I strongly prefer keeping the exit_reason name for the > basic exit reason. The vast majority of VM-Exits do not have modifiers > set, i.e. "basic exit reason" == vmcs.EXIT_REASON for nearly all normal > usage. This holds true in every form of communication, e.g. when discussing > VM-Exit reasons, it's never qualified with "basic", it's simply the exit > reason. IMO the code is better off following the colloquial usage of "exit > reason". A simple comment above the union would suffice to clear up any > confusion with respect to the SDM.
Well, for this reason we can keep exit_reason for 16-bit usage, and define full/extended_exit_reason for 32-bit cases. This makes less code churn.
But after we choose to use exit_reason and full/extended_exit_reason, what if someday new modifier flags are added and we want to enable some modifier flags for nested case? I guess we need to change existing exit_reason to full/extended_exit_reason in nested.c/nested.h to keep the naming rule consistent.
>> Maybe we name it >> >> union { >> u16 basic_exit_reason; >> u32 exit_reason; >> } >> >> as what SDM defines? >> >>> --- a/arch/x86/kvm/vmx/vmx.c >>> +++ b/arch/x86/kvm/vmx/vmx.c >>> @@ -5818,7 +5818,7 @@ static int vmx_handle_exit(struct kvm_vcpu *vcpu, >>> if (is_guest_mode(vcpu) && nested_vmx_exit_reflected(vcpu, exit_reason)) >>> return nested_vmx_reflect_vmexit(vcpu, exit_reason); >>> >>> - if (exit_reason & VMX_EXIT_REASONS_FAILED_VMENTRY) { >>> + if (vmx->full_exit_reason & VMX_EXIT_REASONS_FAILED_VMENTRY) { > > If we do go the union route, this snippet of code is insufficient, the > full/extended exit reason needs to be snapshotted early for use in the > tracepoint and in fail_entry.hardware_entry_failure_reason. > >>> dump_vmcs(); >>> vcpu->run->exit_reason = KVM_EXIT_FAIL_ENTRY; >>> vcpu->run->fail_entry.hardware_entry_failure_reason >>> @@ -6620,11 +6620,12 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu) >>> vmx->nested.nested_run_pending = 0; >>> vmx->idt_vectoring_info = 0; >>> >>> - vmx->exit_reason = vmx->fail ? 0xdead : vmcs_read32(VM_EXIT_REASON); >>> - if ((u16)vmx->exit_reason == EXIT_REASON_MCE_DURING_VMENTRY) >>> + vmx->full_exit_reason = vmx->fail ? 0xdead : vmcs_read32(VM_EXIT_REASON); >>> + if (vmx->exit_reason == EXIT_REASON_MCE_DURING_VMENTRY) >>> kvm_machine_check(); >>> >>> - if (vmx->fail || (vmx->exit_reason & VMX_EXIT_REASONS_FAILED_VMENTRY)) >>> + if (vmx->fail || >>> + (vmx->full_exit_reason & VMX_EXIT_REASONS_FAILED_VMENTRY)) >>> return; >>> >>> vmx->loaded_vmcs->launched = 1; >>> diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h >>> index 7f42cf3dcd70..60c09640ea59 100644 >>> --- a/arch/x86/kvm/vmx/vmx.h >>> +++ b/arch/x86/kvm/vmx/vmx.h >>> @@ -260,7 +260,10 @@ struct vcpu_vmx { >>> int vpid; >>> bool emulation_required; >>> >>> - u32 exit_reason; >>> + union { >>> + u16 exit_reason; >>> + u32 full_exit_reason; >>> + } >>> >>> /* Posted interrupt descriptor */ >>> struct pi_desc pi_desc; >>> >>> >>> >>> >>> >>>> -- >>>> Vitaly >>>> >>
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