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
