linux-kernel - Re: [RESEND PATCH] KVM: x86/mmu: make apf token non-zero to fix bug

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Date:   Thu, 10 Mar 2022 13:09:13 +0800
From:   Xinlong Lin <linxl3@...gsu.com>
To:     "zhangliang (AG)" <zhangliang5@...wei.com>, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        x86@...nel.org
Cc:     pbonzini@...hat.com, seanjc@...gle.com, vkuznets@...hat.com,
        wanpengli@...cent.com, jmattson@...gle.com, joro@...tes.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        wangzhigang17@...wei.com
Subject: Re: [RESEND PATCH] KVM: x86/mmu: make apf token non-zero to fix bug

Thank you and sorry for the noise.

On 2022/3/10 11:42, zhangliang (AG) wrote:
> No. Because '(++vcpu->arch.apf.id << 12)' may also produce zero value.
> 
> On 2022/3/10 11:34, Xinlong Lin wrote:
>>
>>
>> On 2022/2/22 11:12, Liang Zhang wrote:
>>> In current async pagefault logic, when a page is ready, KVM relies on
>>> kvm_arch_can_dequeue_async_page_present() to determine whether to deliver
>>> a READY event to the Guest. This function test token value of struct
>>> kvm_vcpu_pv_apf_data, which must be reset to zero by Guest kernel when a
>>> READY event is finished by Guest. If value is zero meaning that a READY
>>> event is done, so the KVM can deliver another.
>>> But the kvm_arch_setup_async_pf() may produce a valid token with zero
>>> value, which is confused with previous mention and may lead the loss of
>>> this READY event.
>>>
>>> This bug may cause task blocked forever in Guest:
>>>    INFO: task stress:7532 blocked for more than 1254 seconds.
>>>          Not tainted 5.10.0 #16
>>>    "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>>>    task:stress          state:D stack:    0 pid: 7532 ppid:  1409
>>>    flags:0x00000080
>>>    Call Trace:
>>>     __schedule+0x1e7/0x650
>>>     schedule+0x46/0xb0
>>>     kvm_async_pf_task_wait_schedule+0xad/0xe0
>>>     ? exit_to_user_mode_prepare+0x60/0x70
>>>     __kvm_handle_async_pf+0x4f/0xb0
>>>     ? asm_exc_page_fault+0x8/0x30
>>>     exc_page_fault+0x6f/0x110
>>>     ? asm_exc_page_fault+0x8/0x30
>>>     asm_exc_page_fault+0x1e/0x30
>>>    RIP: 0033:0x402d00
>>>    RSP: 002b:00007ffd31912500 EFLAGS: 00010206
>>>    RAX: 0000000000071000 RBX: ffffffffffffffff RCX: 00000000021a32b0
>>>    RDX: 000000000007d011 RSI: 000000000007d000 RDI: 00000000021262b0
>>>    RBP: 00000000021262b0 R08: 0000000000000003 R09: 0000000000000086
>>>    R10: 00000000000000eb R11: 00007fefbdf2baa0 R12: 0000000000000000
>>>    R13: 0000000000000002 R14: 000000000007d000 R15: 0000000000001000
>>>
>>> Signed-off-by: Liang Zhang <zhangliang5@...wei.com>
>>> ---
>>>    arch/x86/kvm/mmu/mmu.c | 13 ++++++++++++-
>>>    1 file changed, 12 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
>>> index 593093b52395..8e24f73bf60b 100644
>>> --- a/arch/x86/kvm/mmu/mmu.c
>>> +++ b/arch/x86/kvm/mmu/mmu.c
>>> @@ -3889,12 +3889,23 @@ static void shadow_page_table_clear_flood(struct kvm_vcpu *vcpu, gva_t addr)
>>>        walk_shadow_page_lockless_end(vcpu);
>>>    }
>>>    +static u32 alloc_apf_token(struct kvm_vcpu *vcpu)
>>> +{
>>> +    /* make sure the token value is not 0 */
>>> +    u32 id = vcpu->arch.apf.id;
>>> +
>>> +    if (id << 12 == 0)
>>> +        vcpu->arch.apf.id = 1;
>>> +
>>> +    return (vcpu->arch.apf.id++ << 12) | vcpu->vcpu_id;
>>> +}
>>> +
>>>    static bool kvm_arch_setup_async_pf(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
>>>                        gfn_t gfn)
>>>    {
>>>        struct kvm_arch_async_pf arch;
>>>    -    arch.token = (vcpu->arch.apf.id++ << 12) | vcpu->vcpu_id;
>> This patch is completely OK. But I have a question, can we simplify it to
>> arch.token = (++vcpu->arch.apf.id << 12) | vcpu->vcpu_id;
>>> +    arch.token = alloc_apf_token(vcpu);
>>>        arch.gfn = gfn;
>>>        arch.direct_map = vcpu->arch.mmu->direct_map;
>>>        arch.cr3 = vcpu->arch.mmu->get_guest_pgd(vcpu);
>>
>> .
>