linux-kernel - Re: [RFC, PATCH 09/12] KVM: TDX: Preallocate PAMT pages to be used in page fault path

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Message-ID: <dtptwhf2si2n2ksz746p67v5oib7h7l7bz57hvxd6rmxne7nht@fvodz653v5nf>
Date: Fri, 30 May 2025 13:28:39 +0300
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To: Chao Gao <chao.gao@...el.com>
Cc: pbonzini@...hat.com, seanjc@...gle.com, rick.p.edgecombe@...el.com, 
	isaku.yamahata@...el.com, kai.huang@...el.com, yan.y.zhao@...el.com, tglx@...utronix.de, 
	mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com, kvm@...r.kernel.org, 
	x86@...nel.org, linux-coco@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [RFC, PATCH 09/12] KVM: TDX: Preallocate PAMT pages to be used
 in page fault path

On Wed, May 14, 2025 at 02:30:34PM +0800, Chao Gao wrote:
> On Fri, May 02, 2025 at 04:08:25PM +0300, Kirill A. Shutemov wrote:
> >Preallocate a page to be used in the link_external_spt() and
> >set_external_spte() paths.
> >
> >In the worst-case scenario, handling a page fault might require a
> >tdx_nr_pamt_pages() pages for each page table level.
> >
> >Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> >---
> > arch/x86/include/asm/kvm_host.h |  2 ++
> > arch/x86/kvm/mmu/mmu.c          | 10 ++++++++++
> > 2 files changed, 12 insertions(+)
> >
> >diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> >index 91958c55f918..a5661499a176 100644
> >--- a/arch/x86/include/asm/kvm_host.h
> >+++ b/arch/x86/include/asm/kvm_host.h
> >@@ -849,6 +849,8 @@ struct kvm_vcpu_arch {
> > 	 */
> > 	struct kvm_mmu_memory_cache mmu_external_spt_cache;
> > 
> >+	struct kvm_mmu_memory_cache pamt_page_cache;
> >+
> > 	/*
> > 	 * QEMU userspace and the guest each have their own FPU state.
> > 	 * In vcpu_run, we switch between the user and guest FPU contexts.
> >diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> >index a284dce227a0..7bfa0dc50440 100644
> >--- a/arch/x86/kvm/mmu/mmu.c
> >+++ b/arch/x86/kvm/mmu/mmu.c
> >@@ -616,6 +616,15 @@ static int mmu_topup_memory_caches(struct kvm_vcpu *vcpu, bool maybe_indirect)
> > 		if (r)
> > 			return r;
> > 	}
> >+
> >+	if (vcpu->kvm->arch.vm_type == KVM_X86_TDX_VM) {
> 
> The check for vcpu->kvm->arch.vm_type == KVM_X86_TDX_VM is identical to
> kvm_has_mirrored_tdp() a few lines above.

Well, yes. But I think it is conceptually different. There can be
different virtualization mode that has mirrored TDP which is not TDX.

> 
> >+		int nr = tdx_nr_pamt_pages(tdx_get_sysinfo());
> 
> Since you're already accessing tdx_sysinfo, you can check if dynamic PAMT is
> enabled and allocate the pamt page cache accordingly.

I will hide it in tdx_nr_pamt_pages() which would return 0 if Dynamic PAMT
is disabled.


-- 
  Kiryl Shutsemau / Kirill A. Shutemov