linux-kernel - Re: [PATCH v2 07/10] KVM: x86: Always populate L1 GPA for KVM_MAP

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Date: Mon, 15 Apr 2024 19:12:07 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "kvm@...r.kernel.org" <kvm@...r.kernel.org>, "Yamahata, Isaku"
	<isaku.yamahata@...el.com>
CC: "seanjc@...gle.com" <seanjc@...gle.com>, "Huang, Kai"
	<kai.huang@...el.com>, "federico.parola@...ito.it"
	<federico.parola@...ito.it>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "isaku.yamahata@...il.com"
	<isaku.yamahata@...il.com>, "dmatlack@...gle.com" <dmatlack@...gle.com>,
	"michael.roth@....com" <michael.roth@....com>, "pbonzini@...hat.com"
	<pbonzini@...hat.com>
Subject: Re: [PATCH v2 07/10] KVM: x86: Always populate L1 GPA for
 KVM_MAP_MEMORY

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

On Wed, 2024-04-10 at 15:07 -0700, isaku.yamahata@...el.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.