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

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Date:	Tue, 28 Jun 2016 10:43:13 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Wanpeng Li <kernellwp@...il.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



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?

Thanks,

Paolo