linux-kernel - Re: [PATCH v2 8/9] KVM: x86, SVM: isolate vcpu->arch.dr6 from vmcb->save.dr6

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Date:   Fri, 8 May 2020 00:33:57 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v2 8/9] KVM: x86, SVM: isolate vcpu->arch.dr6 from
 vmcb->save.dr6

On 07/05/20 21:28, Peter Xu wrote:
>> -	svm->vcpu.arch.dr6 = dr6;
>> +	WARN_ON(svm->vcpu.arch.switch_db_regs & KVM_DEBUGREG_WONT_EXIT);
>> +	svm->vcpu.arch.dr6 &= ~(DR_TRAP_BITS | DR6_RTM);
>> +	svm->vcpu.arch.dr6 |= dr6 & ~DR6_FIXED_1;
> I failed to figure out what the above calculation is going to do... 

The calculation is merging the cause of the #DB with the guest DR6.
It's basically the same effect as kvm_deliver_exception_payload. The
payload has DR6_RTM flipped compared to DR6, so you have the following
simplfications:

	payload = (dr6 ^ DR6_RTM) & ~DR6_FIXED_1;
	/* This is kvm_deliver_exception_payload: */
        vcpu->arch.dr6 &= ~DR_TRAP_BITS;
        vcpu->arch.dr6 |= DR6_RTM;
	/* copy dr6 bits other than RTM */
        vcpu->arch.dr6 |= payload;
	/* copy flipped RTM bit */
        vcpu->arch.dr6 ^= payload & DR6_RTM;

->

	payload = (dr6 ^ DR6_RTM) & ~DR6_FIXED_1;
	/* clear RTM here, so that we can OR it below */
        vcpu->arch.dr6 &= ~(DR_TRAP_BITS | DR6_RTM);
	/* copy dr6 bits other than RTM */
        vcpu->arch.dr6 |= payload & ~DR6_RTM;
	/* copy flipped RTM bit */
        vcpu->arch.dr6 |= (payload ^ DR6_RTM) & DR6_RTM;

->

	/* we can drop the double XOR of DR6_RTM */
	dr6 &= ~DR6_FIXED_1;
        vcpu->arch.dr6 &= ~(DR_TRAP_BITS | DR6_RTM);
        vcpu->arch.dr6 |= dr6 & ~DR6_RTM;
        vcpu->arch.dr6 |= dr6 & DR6_RTM;

->

	/* we can do the two ORs with a single operation */
        vcpu->arch.dr6 &= ~(DR_TRAP_BITS | DR6_RTM);
        vcpu->arch.dr6 |= dr6 & ~DR6_FIXED_1;

> E.g., I
> think the old "BT|BS|BD" bits in the old arch.dr6 cache will be leftover even
> if none of them is set in save.dr6, while we shouldn't?

Those bits should be kept; this is covered for example by the "hw
breakpoint (test that dr6.BS is not cleared)" testcase in kvm-unit-tests
x86/debug.c.

Thanks,

Paolo