linux-kernel - Re: [PATCH] KVM: vmx: fix underflow in TSC deadline calculation

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Date:	Tue, 28 Jun 2016 16:46:29 +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>, Wanpeng Li <wanpeng.li@...mail.com>,
	Yunhong Jiang <yunhong.jiang@...il.com>
Subject: Re: [PATCH] KVM: vmx: fix underflow in TSC deadline calculation

2016-06-28 16:43 GMT+08:00 Paolo Bonzini <pbonzini@...hat.com>:
>
>
> On 28/06/2016 08:15, Wanpeng Li wrote:
>> 2016-06-27 21:11 GMT+08:00 Paolo Bonzini <pbonzini@...hat.com>:
>>> If the TSC deadline timer is programmed really close to the deadline or
>>> even in the past, the computation in vmx_set_hv_timer can underflow and
>>> cause delta_tsc to be set to a huge value.  This generally results
>>> in vmx_set_hv_timer returning -ERANGE, but we can fix it by limiting
>>> delta_tsc to be positive or zero.
>>>
>>> Reported-by: Wanpeng Li <wanpeng.li@...mail.com>
>>> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
>>> ---
>>>  arch/x86/kvm/vmx.c | 6 +++---
>>>  1 file changed, 3 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>>> index c1d655c10fd2..85e2f0a882ca 100644
>>> --- a/arch/x86/kvm/vmx.c
>>> +++ b/arch/x86/kvm/vmx.c
>>> @@ -10829,9 +10829,9 @@ static inline int u64_shl_div_u64(u64 a, unsigned int shift,
>>>  static int vmx_set_hv_timer(struct kvm_vcpu *vcpu, u64 guest_deadline_tsc)
>>>  {
>>>         struct vcpu_vmx *vmx = to_vmx(vcpu);
>>> -       u64 tscl = rdtsc(), delta_tsc;
>>> -
>>> -       delta_tsc = guest_deadline_tsc - kvm_read_l1_tsc(vcpu, tscl);
>>> +       u64 tscl = rdtsc();
>>> +       u64 guest_tscl = kvm_read_l1_tsc(vcpu, tscl);
>>> +       u64 delta_tsc = max(guest_deadline_tsc, guest_tscl) - guest_tscl;
>>>
>>>         /* Convert to host delta tsc if tsc scaling is enabled */
>>>         if (vcpu->arch.tsc_scaling_ratio != kvm_default_tsc_scaling_ratio &&
>>
>> This patch still can't fix the bug after my testing. I have a patch on
>> hand and will send out soon.
>
> Nice!  Do you think we need both patches?

Yeah, we can keep them separately. :)

Regards,
Wanpeng Li