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

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Message-ID: <aLCwpNygeC64Bkra@google.com>
Date: Thu, 28 Aug 2025 12:40:20 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Yan Zhao <yan.y.zhao@...el.com>
Cc: Rick P Edgecombe <rick.p.edgecombe@...el.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>, 
	"pbonzini@...hat.com" <pbonzini@...hat.com>, Vishal Annapurve <vannapurve@...gle.com>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, 
	"michael.roth@....com" <michael.roth@....com>, Ira Weiny <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, Yan Zhao wrote:
> 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

That won't fix this issue though, becuase @fault will be valid and so max_level
will still be KVM_MAX_HUGEPAGE_LEVEL.  Which is by design, the intent in that
flow is that KVM should have gotten the level when getting the pfn from gmem.

IIUC, this particular flow _must_ map at 4KiB, so I think forcing PG_LEVEL_4k is
the right solution.