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

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Date:	Tue, 28 Jun 2016 14:15:45 +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

2016-06-27 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.

Regards,
Wanpeng Li