linux-kernel - Re: [RFC PATCH 02/12] KVM: x86/mmu: Add dedicated API to map guest

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Message-ID: <aK/1+Al99CoTKzKH@yzhao56-desk.sh.intel.com>
Date: Thu, 28 Aug 2025 14:23:52 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
CC: "seanjc@...gle.com" <seanjc@...gle.com>, "kvm@...r.kernel.org"
	<kvm@...r.kernel.org>, "pbonzini@...hat.com" <pbonzini@...hat.com>,
	"Annapurve, Vishal" <vannapurve@...gle.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "michael.roth@....com"
	<michael.roth@....com>, "Weiny, Ira" <ira.weiny@...el.com>
Subject: Re: [RFC PATCH 02/12] KVM: x86/mmu: Add dedicated API to map
 guest_memfd pfn into TDP MMU

On Thu, Aug 28, 2025 at 09:26:50AM +0800, Edgecombe, Rick P wrote:
> On Wed, 2025-08-27 at 17:54 -0700, Rick Edgecombe wrote:
> > > 
> > > Then, what about setting
> > >                 .max_level = PG_LEVEL_4K,
> > > directly?
> > > 
> > > Otherwise, the "(KVM_BUG_ON(level != PG_LEVEL_4K, kvm)" would be triggered
> > > in
> > > tdx_sept_set_private_spte().
> > 
> > Yes this fails to boot a TD. With max_level = PG_LEVEL_4K it passes the full
> > tests. I don't think it's ideal to encode PAGE.ADD details here though.
> > 
> > But I'm not immediately clear what is going wrong. The old struct
> > kvm_page_fault
> > looks pretty similar. Did you root cause it?
>
> Oh, duh. Because we are passing in the PFN now so it can't know the size. So
> it's not about PAGE.ADD actually.
Right, it's because the previous kvm_tdp_map_page() updates fault->max_level in
kvm_mmu_faultin_pfn_private() by checking the private_max_mapping_level hook.

However, private_max_mapping_level() skips the faultin step and goes straight
to kvm_tdp_mmu_map().

> Sill, how about calling the function kvm_tdp_mmu_map_private_pfn_4k(), or
> passing in the level?
Looks [1] can also address this issue. Not sure which one Sean prefers.

[1] https://lore.kernel.org/all/20250729225455.670324-15-seanjc@google.com