linux-kernel - Re: [PATCH] KVM: x86: fix missed hardware breakpoints

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Message-Id: <6DD9380A-1EF7-4226-9405-017D17859472@gmail.com>
Date:	Fri, 26 Feb 2016 03:28:33 -0800
From:	Nadav Amit <nadav.amit@...il.com>
To:	Xiao Guangrong <guangrong.xiao@...ux.intel.com>
Cc:	Paolo Bonzini <pbonzini@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	kvm <kvm@...r.kernel.org>, oleg@...hat.com,
	Nadav Amit <namit@...technion.ac.il>, avagin@...il.com,
	stable@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: fix missed hardware breakpoints

Xiao Guangrong <guangrong.xiao@...ux.intel.com> wrote:

> On 02/19/2016 06:56 PM, Paolo Bonzini wrote:
>> Sometimes when setting a breakpoint a process doesn't stop on it.
>> This is because the debug registers are not loaded correctly on
>> VCPU load.
>> 
>> 
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index 4244c2baf57d..f4891f2ece23 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -2752,6 +2752,7 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>>  	}
>> 
>>  	kvm_make_request(KVM_REQ_STEAL_UPDATE, vcpu);
>> +	vcpu->arch.switch_db_regs |= KVM_DEBUGREG_RELOAD;
> 
> Er, i do not understand how it works. The BP is enabled in this test case so
> the debug registers are always reloaded before entering guest as
> KVM_DEBUGREG_BP_ENABLED bit is always set on switch_db_regs. What did i miss?

Note that KVM_DEBUGREG_BP_ENABLED does not have to be set, since
kvm_update_dr7() is not called once the guest has KVM_DEBUGREG_WONT_EXIT.

Nadav