linux-kernel - Re: [RFC PATCH 10/21] KVM: x86/mmu: Disallow page merging (huge page adjustment) for mirror root

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Message-ID: <20fd95c417229018a8dfb8f3a50ba6a3bcf53e6c.camel@intel.com>
Date: Fri, 16 May 2025 17:50:53 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "Zhao, Yan Y" <yan.y.zhao@...el.com>
CC: "kvm@...r.kernel.org" <kvm@...r.kernel.org>, "Li, Xiaoyao"
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Subject: Re: [RFC PATCH 10/21] KVM: x86/mmu: Disallow page merging (huge page
 adjustment) for mirror root

On Fri, 2025-05-16 at 12:01 +0800, Yan Zhao wrote:
> > Maybe we should rename nx_huge_page_workaround_enabled to something more
> > generic
> > and do the is_mirror logic in kvm_mmu_do_page_fault() when setting it. It
> > should
> > shrink the diff and centralize the logic.
> Hmm, I'm reluctant to rename nx_huge_page_workaround_enabled, because
> 
> (1) Invoking disallowed_hugepage_adjust() for mirror root is to disable page
>     promotion for TDX private memory, so is only applied to TDP MMU.
> (2) nx_huge_page_workaround_enabled is used specifically for nx huge pages.
>     fault->huge_page_disallowed = fault->exec && fault-
> >nx_huge_page_workaround_enabled;

Oh, good point.

> 
>     if (fault->huge_page_disallowed)
>         account_nx_huge_page(vcpu->kvm, sp,
>                              fault->req_level >= it.level);
>     
>     sp->nx_huge_page_disallowed = fault->huge_page_disallowed.
> 
>     Affecting fault->huge_page_disallowed would impact
>     sp->nx_huge_page_disallowed as well and would disable huge pages entirely.
> 
>     So, we still need to keep nx_huge_page_workaround_enabled.
> 
> If we introduce a new flag fault->disable_hugepage_adjust, and set it in
> kvm_mmu_do_page_fault(), we would also need to invoke
> tdp_mmu_get_root_for_fault() there as well.
> 
> Checking for mirror root for non-TDX VMs is not necessary, and the invocation
> of
> tdp_mmu_get_root_for_fault() seems redundant with the one in
> kvm_tdp_mmu_map().

Also true. What about a wrapper for MMU code to check instead of fault-
>nx_huge_page_workaround_enabled then?

Also, why not check is_mirror_sp() in disallowed_hugepage_adjust() instead of
passing in an is_mirror arg?

There must be a way to have it fit in better with disallowed_hugepage_adjust()
without adding so much open coded boolean logic.