Re: [PATCH v2 07/10] KVM: x86: Always populate L1 GPA for KVM_MAP_MEMORY

From: "Edgecombe, Rick P" <rick.p.edgecombe@intel.com>
To: "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
	"Yamahata, Isaku" <isaku.yamahata@intel.com>
Cc: "seanjc@google.com" <seanjc@google.com>,
	"Huang, Kai" <kai.huang@intel.com>,
	"federico.parola@polito.it" <federico.parola@polito.it>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"isaku.yamahata@gmail.com" <isaku.yamahata@gmail.com>,
	"dmatlack@google.com" <dmatlack@google.com>,
	"michael.roth@amd.com" <michael.roth@amd.com>,
	"pbonzini@redhat.com" <pbonzini@redhat.com>
Subject: Re: [PATCH v2 07/10] KVM: x86: Always populate L1 GPA for KVM_MAP_MEMORY
Date: Mon, 15 Apr 2024 19:12:07 +0000	[thread overview]
Message-ID: <116179545fafbf39ed01e1f0f5ac76e0467fc09a.camel@intel.com> (raw)
In-Reply-To: <2f1de1b7b6512280fae4ac05e77ced80a585971b.1712785629.git.isaku.yamahata@intel.com>

I wouldn't call myself much of an expert on nested, but...

On Wed, 2024-04-10 at 15:07 -0700, isaku.yamahata@intel.com wrote:
> There are several options to populate L1 GPA irrelevant to vCPU mode.
> - Switch vCPU MMU only: This patch.
>   Pros: Concise implementation.
>   Cons: Heavily dependent on the KVM MMU implementation.

Is switching just the MMU enough here? Won't the MTRRs and other vcpu bits be
wrong?

> - Use kvm_x86_nested_ops.get/set_state() to switch to/from guest mode.
>   Use __get/set_sregs2() to switch to/from SMM mode.
>   Pros: straightforward.
>   Cons: This may cause unintended side effects.

Cons make sense.

> - Refactor KVM page fault handler not to pass vCPU. Pass around necessary
>   parameters and struct kvm.
>   Pros: The end result will have clearly no side effects.
>   Cons: This will require big refactoring.

But doesn't the fault handler need the vCPU state?

> - Return error on guest mode or SMM mode:  Without this patch.
>   Pros: No additional patch.
>   Cons: Difficult to use.

Hmm... For the non-TDX use cases this is just an optimization, right? For TDX
there shouldn't be an issue. If so, maybe this last one is not so horrible.