Re: [PATCH kernel 5/9] KVM: SVM/SEV/SEV-ES: Rework intercepts

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On 1/7/23 07:49, Sean Christopherson wrote:
On Thu, Jun 15, 2023, Alexey Kardashevskiy wrote:
@@ -2976,6 +2977,16 @@ static void sev_es_init_vmcb(struct vcpu_svm *svm)
  	svm_set_intercept(svm, TRAP_CR4_WRITE);
  	svm_set_intercept(svm, TRAP_CR8_WRITE);
+ /*
+	 * DR7 access must remain intercepted for an SEV-ES guest to disallow
+	 * the guest kernel set up a #DB on memory that's needed to vector a #DB
+	 * as otherwise the CPU gets stuck in an infinite #DB loop.
+	 */

This isn't correct.  Letting the guest configuring breakpoints would be weird
and nonsensical, but it wouldn't lead to infinite #DBs so long as KVM intercepts
#DB.

True. OTOH not intercepting #DB and intercepting only DR7 would do the same thing.

KVM intercepts DR7 when DebugSwap isn't enabled because otherwise KVM has no way
of context switching DR[0-3] for the guest.  At least, I assume that's the case,
AFAICT the APM never actually says what happens with DR[0-3] when DebugSwap is
disabled.

This is the SEV-ES code, no DR[0-3] context switching anyway, is not it?

The actual immediate reason for intercepting DR7 is "SEV-ES GHCB" but this does not really explain it to me :-/

4.5 Debug Register Support
Currently, hardware debug traps are not supported for an SEV-ES guest. The hypervisor must set the intercept for both read and write of the debug control register (DR7). With the intercepts in place, the #VC handler will be invoked when the guest accesses DR7. For a write to DR7, the #VC handler should perform Standard VMGExit processing. The #VC handler must not update the actual DR7 register, but rather it should cache the DR7 value being written. For a read of DR7, the #VC handler should return the cached value of the DR7 register.


--
Alexey



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