Re: [PATCH 2/2] KVM: x86/mmu: KVM: x86/mmu: Skip unsync when large pages are allowed

[Sean Christopherson <seanjc@google.com>]

On Thu, Mar 12, 2026, Sean Christopherson wrote:
> On Fri, Jan 23, 2026, Lai Jiangshan wrote:
> This is what I have locally, please holler if you object to landing the code
> after the write-tracked check.
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 363967a17069..3d0e0c1b5332 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -2940,6 +2940,15 @@ int mmu_try_to_unsync_pages(struct kvm *kvm, const struct kvm_memory_slot *slot,
>         if (kvm_gfn_is_write_tracked(kvm, slot, gfn))
>                 return -EPERM;
>  
> +       /*
> +        * Only 4KiB mappings can become unsync, and KVM disallows hugepages
> +        * for unsync gfns.  Upper-level gPTEs (leaf or non-leaf) are always
> +        * write-protected (see above), thus if the gfn can be mapped with a
> +        * hugepage and isn't write-tracked, it can't be unsync.

Gah, I swapped the ordering of who is doing what.  The comment should be this:

	/*
	 * Only 4KiB mappings can become unsync, and KVM disallows hugepages
	 * when accounting 4KiB shadow pages.  Upper-level gPTEs are always
	 * write-protected (see above), thus if the gfn can be mapped with a
	 * hugepage and isn't write-tracked, it can't have a shadow page.
	 */
	if (!lpage_info_slot(gfn, slot, PG_LEVEL_2M)->disallow_lpage)
		return 0;

> +        */
> +       if (!lpage_info_slot(gfn, slot, PG_LEVEL_2M)->disallow_lpage)
> +               return 0;
> +
>         /*
>          * The page is not write-tracked, mark existing shadow pages unsync
>          * unless KVM is synchronizing an unsync SP.  In that case, KVM must
> 
> 
> >  	/*
> >  	 * Force write-protection if the page is being tracked.  Note, the page
> >  	 * track machinery is used to write-protect upper-level shadow pages,
> > -- 
> > 2.19.1.6.gb485710b
> >