linux-kernel - Re: [PATCH 03/16] KVM: x86/tdp_mmu: Add a helper function to walk down the TDP MMU

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Date: Fri, 17 May 2024 02:08:34 -0700
From: Isaku Yamahata <isaku.yamahata@...el.com>
To: Chao Gao <chao.gao@...el.com>
Cc: Rick Edgecombe <rick.p.edgecombe@...el.com>, pbonzini@...hat.com,
	seanjc@...gle.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, isaku.yamahata@...il.com,
	erdemaktas@...gle.com, sagis@...gle.com, yan.y.zhao@...el.com,
	dmatlack@...gle.com
Subject: Re: [PATCH 03/16] KVM: x86/tdp_mmu: Add a helper function to walk
 down the TDP MMU

On Fri, May 17, 2024 at 03:44:27PM +0800,
Chao Gao <chao.gao@...el.com> wrote:

> On Tue, May 14, 2024 at 05:59:39PM -0700, Rick Edgecombe wrote:
> >From: Isaku Yamahata <isaku.yamahata@...el.com>
> >
> >Export a function to walk down the TDP without modifying it.
> >
> >Future changes will support pre-populating TDX private memory. In order to
> >implement this KVM will need to check if a given GFN is already
> >pre-populated in the mirrored EPT, and verify the populated private memory
> >PFN matches the current one.[1]
> >
> >There is already a TDP MMU walker, kvm_tdp_mmu_get_walk() for use within
> >the KVM MMU that almost does what is required. However, to make sense of
> >the results, MMU internal PTE helpers are needed. Refactor the code to
> >provide a helper that can be used outside of the KVM MMU code.
> >
> >Refactoring the KVM page fault handler to support this lookup usage was
> >also considered, but it was an awkward fit.
> >
> >Link: https://lore.kernel.org/kvm/ZfBkle1eZFfjPI8l@google.com/ [1]
> >Signed-off-by: Isaku Yamahata <isaku.yamahata@...el.com>
> >Signed-off-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
> >---
> >This helper will be used in the future change that implements
> >KVM_TDX_INIT_MEM_REGION. Please refer to the following commit for the
> >usage:
> >https://github.com/intel/tdx/commit/2832c6d87a4e6a46828b193173550e80b31240d4
> >
> >TDX MMU Part 1:
> > - New patch
> >---
> > arch/x86/kvm/mmu.h         |  3 +++
> > arch/x86/kvm/mmu/tdp_mmu.c | 37 +++++++++++++++++++++++++++++++++----
> > 2 files changed, 36 insertions(+), 4 deletions(-)
> >
> >diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
> >index dc80e72e4848..3c7a88400cbb 100644
> >--- a/arch/x86/kvm/mmu.h
> >+++ b/arch/x86/kvm/mmu.h
> >@@ -275,6 +275,9 @@ extern bool tdp_mmu_enabled;
> > #define tdp_mmu_enabled false
> > #endif
> > 
> >+int kvm_tdp_mmu_get_walk_private_pfn(struct kvm_vcpu *vcpu, u64 gpa,
> >+				     kvm_pfn_t *pfn);
> >+
> > static inline bool kvm_memslots_have_rmaps(struct kvm *kvm)
> > {
> > 	return !tdp_mmu_enabled || kvm_shadow_root_allocated(kvm);
> >diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> >index 1259dd63defc..1086e3b2aa5c 100644
> >--- a/arch/x86/kvm/mmu/tdp_mmu.c
> >+++ b/arch/x86/kvm/mmu/tdp_mmu.c
> >@@ -1772,16 +1772,14 @@ bool kvm_tdp_mmu_write_protect_gfn(struct kvm *kvm,
> >  *
> >  * Must be called between kvm_tdp_mmu_walk_lockless_{begin,end}.
> >  */
> >-int kvm_tdp_mmu_get_walk(struct kvm_vcpu *vcpu, u64 addr, u64 *sptes,
> >-			 int *root_level)
> >+static int __kvm_tdp_mmu_get_walk(struct kvm_vcpu *vcpu, u64 addr, u64 *sptes,
> >+				  bool is_private)
> 
> is_private isn't used.
> 
> > {
> > 	struct tdp_iter iter;
> > 	struct kvm_mmu *mmu = vcpu->arch.mmu;
> > 	gfn_t gfn = addr >> PAGE_SHIFT;
> > 	int leaf = -1;
> > 
> >-	*root_level = vcpu->arch.mmu->root_role.level;
> >-
> > 	tdp_mmu_for_each_pte(iter, mmu, gfn, gfn + 1) {
> > 		leaf = iter.level;
> > 		sptes[leaf] = iter.old_spte;
> >@@ -1790,6 +1788,37 @@ int kvm_tdp_mmu_get_walk(struct kvm_vcpu *vcpu, u64 addr, u64 *sptes,
> > 	return leaf;
> > }
> > 
> >+int kvm_tdp_mmu_get_walk(struct kvm_vcpu *vcpu, u64 addr, u64 *sptes,
> >+			 int *root_level)
> >+{
> >+	*root_level = vcpu->arch.mmu->root_role.level;
> >+
> >+	return __kvm_tdp_mmu_get_walk(vcpu, addr, sptes, false);
> >+}
> >+
> >+int kvm_tdp_mmu_get_walk_private_pfn(struct kvm_vcpu *vcpu, u64 gpa,
> >+				     kvm_pfn_t *pfn)
> 
> private_pfn probably is a misnomer. shared/private is an attribute of
> GPA rather than pfn. Since the function is to get pfn from gpa, how about
> kvm_tdp_mmu_gpa_to_pfn()?
> 
> And the function is limited to handle private gpa only. It is an artificial
> limitation we can get rid of easily. e.g., by making the function take
> "is_private" boolean and relay it to __kvm_tdp_mmu_get_walk(). I know TDX
> just calls the function to convert private gpa but having a generic API
> can accommodate future use cases (e.g., get hpa from shared gpa) w/o the
> need of refactoring.

Agreed.  Based on a patch at [1], we can have something like
int kvm_tdp_mmu_get_walk(struct kvm_vcpu *vcpu, u64 gpa,
                         enum kvm_tdp_mmu_root_types root_type,
                         kvm_pfn_t *pfn);


[1] https://lore.kernel.org/kvm/55c24448fdf42d383d45601ff6c0b07f44f61787.camel@intel.com/
-- 
Isaku Yamahata <isaku.yamahata@...el.com>