path: root/virt
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authorPaolo Bonzini <pbonzini@redhat.com>2020-05-06 06:40:04 -0400
committerPaolo Bonzini <pbonzini@redhat.com>2020-05-08 07:44:31 -0400
commitd67668e9dd76d98136048935723947156737932b (patch)
treec83fef9b2b749e7409372e35d068f7529f10423a /virt
parent5679b803e44ed8947e8c2a7f44cdef1d93ea24d5 (diff)
KVM: x86, SVM: isolate vcpu->arch.dr6 from vmcb->save.dr6
There are two issues with KVM_EXIT_DEBUG on AMD, whose root cause is the different handling of DR6 on intercepted #DB exceptions on Intel and AMD. On Intel, #DB exceptions transmit the DR6 value via the exit qualification field of the VMCS, and the exit qualification only contains the description of the precise event that caused a vmexit. On AMD, instead the DR6 field of the VMCB is filled in as if the #DB exception was to be injected into the guest. This has two effects when guest debugging is in use: * the guest DR6 is clobbered * the kvm_run->debug.arch.dr6 field can accumulate more debug events, rather than just the last one that happened (the testcase in the next patch covers this issue). This patch fixes both issues by emulating, so to speak, the Intel behavior on AMD processors. The important observation is that (after the previous patches) the VMCB value of DR6 is only ever observable from the guest is KVM_DEBUGREG_WONT_EXIT is set. Therefore we can actually set vmcb->save.dr6 to any value we want as long as KVM_DEBUGREG_WONT_EXIT is clear, which it will be if guest debugging is enabled. Therefore it is possible to enter the guest with an all-zero DR6, reconstruct the #DB payload from the DR6 we get at exit time, and let kvm_deliver_exception_payload move the newly set bits into vcpu->arch.dr6. Some extra bits may be included in the payload if KVM_DEBUGREG_WONT_EXIT is set, but this is harmless. This may not be the most optimized way to deal with this, but it is simple and, being confined within SVM code, it gets rid of the set_dr6 callback and kvm_update_dr6. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
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