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Date: Thu, 8 Jun 2017 20:32:14 +0800
From: Wanpeng Li <kernellwp@...il.com>
To: Paolo Bonzini <pbonzini@...hat.com>
Cc: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
kvm <kvm@...r.kernel.org>,
Radim Krčmář <rkrcmar@...hat.com>,
Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [PATCH RFC] KVM: async_pf: fix async_pf exception injection
2017-06-08 19:52 GMT+08:00 Paolo Bonzini <pbonzini@...hat.com>:
>
>
> On 08/06/2017 11:30, Wanpeng Li wrote:
>> However, I found that "nr == PF_VECTOR && vmx->apf_reason != 0" never be true
>> in nested_vmx_check_exception(). SVM depends on the similar stuff in
>> nested_svm_intercept() which makes me confusing how it can works. In addition,
>> vmx/svm->apf_reason should be got in L1 since apf_reason.reason will make sense
>> just in pv guest. So vmx/svm->apf_reason should always be 0 on L0.
>
> I agree. My suggestion is a series like this:
>
> 1) remove all arguments except the first in
> kvm_x86_ops->queue_exception: they can extract the arguments from
> vcpu->arch.exception themselves
>
> 2) do the same in nested_{vmx,svm}_check_exception
>
> 3) add an async_page_fault member to vcpu->arch.exception
Agreed.
>
>> I change the
>> condition to "nr == PF_VECTOR && error_code == 0" to intercept async_pf, however,
>> the below bug will be splatted:
>
> Right, because error_code == 0 is a valid error code.
>
> For stable releases, this should be enough:
We should also take care PAGE_READY async_pfs injection. I will send a
patch to avoid async pf injection stuff in guest mode for stable
release.
>
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index ac7810513d0e..c5f0023bac2d 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -3689,6 +3689,9 @@ static bool can_do_async_pf(struct kvm_vcpu *vcpu)
> kvm_event_needs_reinjection(vcpu)))
> return false;
>
> + if (is_guest_mode(vcpu))
> + return false;
> +
> return kvm_x86_ops->interrupt_allowed(vcpu);
> }
>
>
> For proper nested async page fault support, you have to be careful,
> because old KVM as L1 you might crash due to the BUG(enable_ept); my
> suggestion is to add another flag bit to MSR_KVM_ASYNC_PF_EN.
>
> If bit 2 is 1, async page faults are delivered to L1 as #PF vmexits; if
> bit 2 is 0, can_do_async_pf returns 0 if in guest mode. When bit 2 is
> 1, async page fault exceptions override the usual exception/intercept
> bitmaps, i.e. they always cause #PF vmexits. What do you think?
Agreed.
>
> Thanks,
>
> Paolo
>
>> BUG: unable to handle kernel paging request at ffffe305770a87e0
>> IP: kfree+0x6f/0x300
>> PGD 0
>> P4D 0
>>
>> Oops: 0000 [#1] PREEMPT SMP
>> CPU: 3 PID: 2187 Comm: transhuge-stres Tainted: G OE 4.12.0-rc4+ #9
>> task: ffff8a9214b58000 task.stack: ffffb46bc34e4000
>> RIP: 0010:kfree+0x6f/0x300
>> RSP: 0000:ffffb46bc34e7b28 EFLAGS: 00010086
>> RAX: ffffe305770a87c0 RBX: ffffb46bc2a1fe70 RCX: 0000000000000001
>> RDX: 0000757180000000 RSI: 00000000ffffffff RDI: 0000000000000096
>> RBP: ffffb46bc34e7b50 R08: 0000000000000000 R09: 0000000000000001
>> R10: ffffb46bc34e7ac8 R11: 68b9962a00000000 R12: 000000a7770a87c0
>> R13: ffffffff90059b75 R14: ffffffff913466c0 R15: ffffe25e06f18000
>> FS: 00007f1904ae7700(0000) GS:ffff8a921a800000(0000) knlGS:0000000000000000
>> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> CR2: ffffe305770a87e0 CR3: 000000040eb5c000 CR4: 00000000001426e0
>> Call Trace:
>> kvm_async_pf_task_wait+0xd5/0x280
>> ? __this_cpu_preempt_check+0x13/0x20
>> do_async_page_fault+0x77/0xb0
>> ? do_async_page_fault+0x77/0xb0
>> async_page_fault+0x28/0x30
>>
>> In additon, if svm->apf_reason doen't make sense on L0, then maybe it also will not
>> work in the function nested_svm_exit_special().
>>
>> The patch below is uncompleted, and your inputs to improve it is a great appreciated.
>
> The patch makes sense. Let's try to make more code common to VMX and
> SVM too once the above is clarified.
Agreed.
Wanpeng Li
>
> Paolo
>
>> Cc: Paolo Bonzini <pbonzini@...hat.com>
>> Cc: Radim Krčmář <rkrcmar@...hat.com>
>> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
>> ---
>> arch/x86/kvm/vmx.c | 41 ++++++++++++++++++++++++++++++++---------
>> 1 file changed, 32 insertions(+), 9 deletions(-)
>>
>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>> index ca5d2b9..21a1b44 100644
>> --- a/arch/x86/kvm/vmx.c
>> +++ b/arch/x86/kvm/vmx.c
>> @@ -616,6 +616,7 @@ struct vcpu_vmx {
>> bool emulation_required;
>>
>> u32 exit_reason;
>> + u32 apf_reason;
>>
>> /* Posted interrupt descriptor */
>> struct pi_desc pi_desc;
>> @@ -2418,11 +2419,12 @@ static void skip_emulated_instruction(struct kvm_vcpu *vcpu)
>> * KVM wants to inject page-faults which it got to the guest. This function
>> * checks whether in a nested guest, we need to inject them to L1 or L2.
>> */
>> -static int nested_vmx_check_exception(struct kvm_vcpu *vcpu, unsigned nr)
>> +static int nested_vmx_check_exception(struct kvm_vcpu *vcpu, unsigned nr, u32 error_code)
>> {
>> struct vmcs12 *vmcs12 = get_vmcs12(vcpu);
>>
>> - if (!(vmcs12->exception_bitmap & (1u << nr)))
>> + if (!((vmcs12->exception_bitmap & (1u << nr)) ||
>> + (nr == PF_VECTOR && error_code == 0)))
>> return 0;
>>
>> nested_vmx_vmexit(vcpu, EXIT_REASON_EXCEPTION_NMI,
>> @@ -2439,7 +2441,7 @@ static void vmx_queue_exception(struct kvm_vcpu *vcpu, unsigned nr,
>> u32 intr_info = nr | INTR_INFO_VALID_MASK;
>>
>> if (!reinject && is_guest_mode(vcpu) &&
>> - nested_vmx_check_exception(vcpu, nr))
>> + nested_vmx_check_exception(vcpu, nr, error_code))
>> return;
>>
>> if (has_error_code) {
>> @@ -5646,14 +5648,31 @@ static int handle_exception(struct kvm_vcpu *vcpu)
>> }
>>
>> if (is_page_fault(intr_info)) {
>> - /* EPT won't cause page fault directly */
>> - BUG_ON(enable_ept);
>> cr2 = vmcs_readl(EXIT_QUALIFICATION);
>> - trace_kvm_page_fault(cr2, error_code);
>> + switch (vmx->apf_reason) {
>> + default:
>> + /* EPT won't cause page fault directly */
>> + BUG_ON(enable_ept);
>> + trace_kvm_page_fault(cr2, error_code);
>>
>> - if (kvm_event_needs_reinjection(vcpu))
>> - kvm_mmu_unprotect_page_virt(vcpu, cr2);
>> - return kvm_mmu_page_fault(vcpu, cr2, error_code, NULL, 0);
>> + if (kvm_event_needs_reinjection(vcpu))
>> + kvm_mmu_unprotect_page_virt(vcpu, cr2);
>> + return kvm_mmu_page_fault(vcpu, cr2, error_code, NULL, 0);
>> + break;
>> + case KVM_PV_REASON_PAGE_NOT_PRESENT:
>> + vmx->apf_reason = 0;
>> + local_irq_disable();
>> + kvm_async_pf_task_wait(cr2);
>> + local_irq_enable();
>> + break;
>> + case KVM_PV_REASON_PAGE_READY:
>> + vmx->apf_reason = 0;
>> + local_irq_disable();
>> + kvm_async_pf_task_wake(cr2);
>> + local_irq_enable();
>> + break;
>> + }
>> + return 0;
>> }
>>
>> ex_no = intr_info & INTR_INFO_VECTOR_MASK;
>> @@ -8600,6 +8619,10 @@ static void vmx_complete_atomic_exit(struct vcpu_vmx *vmx)
>> vmx->exit_intr_info = vmcs_read32(VM_EXIT_INTR_INFO);
>> exit_intr_info = vmx->exit_intr_info;
>>
>> + /* if exit due to PF check for async PF */
>> + if (is_page_fault(exit_intr_info))
>> + vmx->apf_reason = kvm_read_and_reset_pf_reason();
>> +
>> /* Handle machine checks before interrupts are enabled */
>> if (is_machine_check(exit_intr_info))
>> kvm_machine_check();
>>
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