[PATCH v4 11/20] KVM: x86/mmu: Allow for NULL vcpu pointer in __kvm_mmu_get_shadow_page()
David Matlack
dmatlack at google.com
Mon May 9 16:59:08 PDT 2022
On Mon, May 9, 2022 at 3:57 PM Sean Christopherson <seanjc at google.com> wrote:
>
> On Mon, May 09, 2022, David Matlack wrote:
> > On Thu, May 5, 2022 at 4:33 PM Sean Christopherson <seanjc at google.com> wrote:
> > >
> > > On Fri, Apr 22, 2022, David Matlack wrote:
> > > > Allow the vcpu pointer in __kvm_mmu_get_shadow_page() to be NULL. Rename
> > > > it to vcpu_or_null to prevent future commits from accidentally taking
> > > > dependency on it without first considering the NULL case.
> > > >
> > > > The vcpu pointer is only used for syncing indirect shadow pages in
> > > > kvm_mmu_find_shadow_page(). A vcpu pointer it not required for
> > > > correctness since unsync pages can simply be zapped. But this should
> > > > never occur in practice, since the only use-case for passing a NULL vCPU
> > > > pointer is eager page splitting which will only request direct shadow
> > > > pages (which can never be unsync).
> > > >
> > > > Even though __kvm_mmu_get_shadow_page() can gracefully handle a NULL
> > > > vcpu, add a WARN() that will fire if __kvm_mmu_get_shadow_page() is ever
> > > > called to get an indirect shadow page with a NULL vCPU pointer, since
> > > > zapping unsync SPs is a performance overhead that should be considered.
> > > >
> > > > Signed-off-by: David Matlack <dmatlack at google.com>
> > > > ---
> > > > arch/x86/kvm/mmu/mmu.c | 40 ++++++++++++++++++++++++++++++++--------
> > > > 1 file changed, 32 insertions(+), 8 deletions(-)
> > > >
> > > > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > > > index 04029c01aebd..21407bd4435a 100644
> > > > --- a/arch/x86/kvm/mmu/mmu.c
> > > > +++ b/arch/x86/kvm/mmu/mmu.c
> > > > @@ -1845,16 +1845,27 @@ static void kvm_mmu_commit_zap_page(struct kvm *kvm,
> > > > &(_kvm)->arch.mmu_page_hash[kvm_page_table_hashfn(_gfn)]) \
> > > > if ((_sp)->gfn != (_gfn) || (_sp)->role.direct) {} else
> > > >
> > > > -static int kvm_sync_page(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
> > > > - struct list_head *invalid_list)
> > > > +static int __kvm_sync_page(struct kvm *kvm, struct kvm_vcpu *vcpu_or_null,
> > > > + struct kvm_mmu_page *sp,
> > > > + struct list_head *invalid_list)
> > > > {
> > > > - int ret = vcpu->arch.mmu->sync_page(vcpu, sp);
> > > > + int ret = -1;
> > > > +
> > > > + if (vcpu_or_null)
> > >
> > > This should never happen. I like the idea of warning early, but I really don't
> > > like that the WARN is far removed from the code that actually depends on @vcpu
> > > being non-NULL. Case in point, KVM should have bailed on the WARN and never
> > > reached this point. And the inner __kvm_sync_page() is completely unnecessary.
> >
> > Yeah that's fair.
> >
> > >
> > > I also don't love the vcpu_or_null terminology; I get the intent, but it doesn't
> > > really help because understand why/when it's NULL.
> >
> > Eh, I don't think it needs to encode why or when. It just needs to
> > flag to the reader (and future code authors) that this vcpu pointer
> > (unlike all other vcpu pointers in KVM) is NULL in certain cases.
>
> My objection is that without the why/when, developers that aren't familiar with
> this code won't know the rules for using vcpu_or_null. E.g. I don't want to end
> up with
>
> if (vcpu_or_null)
> do x;
> else
> do y;
>
> because inevitably it'll become unclear whether or not that code is actually _correct_.
> It might not #GP on a NULL pointer, but it doesn't mean it's correct.
Ah, right. And that's actually why I put the big comment and WARN in
__kvm_mmu_get_shadow_page(). Readers could easily jump to where
vcpu_or_null is passed in and see the rules around it. But if we move
the WARN to the kvm_sync_page() call, I agree it will be harder for
readers to know the rules and "vcpu_or_null" starts to become a risky
variable.
>
> > > I played around with casting, e.g. to/from an unsigned long or void *, to prevent
> > > usage, but that doesn't work very well because 'unsigned long' ends up being
> > > awkward/confusing, and 'void *' is easily lost on a function call. And both
> > > lose type safety :-(
> >
> > Yet another shortcoming of C :(
>
> And lack of closures, which would work very well here.
>
> > (The other being our other discussion about the RET_PF* return codes
> > getting easily misinterpreted as KVM's magic return-to-user /
> > continue-running-guest return codes.)
> >
> > Makes me miss Rust!
> >
> > >
> > > All in all, I think I'd prefer this patch to simply be a KVM_BUG_ON() if
> > > kvm_mmu_find_shadow_page() encounters an unsync page. Less churn, and IMO there's
> > > no real loss in robustness, e.g. we'd really have to screw up code review and
> > > testing to introduce a null vCPU pointer dereference in this code.
> >
> > Agreed about moving the check here and dropping __kvm_sync_page(). But
> > I would prefer to retain the vcpu_or_null name (or at least something
> > other than "vcpu" to indicate there's something non-standard about
> > this pointer).
>
> The least awful idea I've come up with is wrapping the vCPU in a struct, e.g.
>
> struct sync_page_info {
> void *vcpu;
> }
>
> That provides the contextual information I want, and also provides the hint that
> something is odd about the vcpu, which you want. It's like a very poor man's closure :-)
>
> The struct could even be passed by value to avoid the miniscule overhead, and to
> make readers look extra hard because it's that much more wierd.
Interesting idea. I'll give it a shot.
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 3d102522804a..068be77a4fff 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -2003,8 +2003,13 @@ static void clear_sp_write_flooding_count(u64 *spte)
> __clear_sp_write_flooding_count(sptep_to_sp(spte));
> }
>
> +/* Wrapper to make it difficult to dereference a potentially NULL @vcpu. */
> +struct sync_page_info {
> + void *vcpu;
> +};
> +
> static struct kvm_mmu_page *kvm_mmu_find_shadow_page(struct kvm *kvm,
> - struct kvm_vcpu *vcpu,
> + struct sync_page_info spi,
> gfn_t gfn,
> struct hlist_head *sp_list,
> union kvm_mmu_page_role role)
> @@ -2041,6 +2046,13 @@ static struct kvm_mmu_page *kvm_mmu_find_shadow_page(struct kvm *kvm,
> goto out;
>
> if (sp->unsync) {
> + /*
> + * Getting indirect shadow pages without a valid @spi
> + * is not supported, i.e. this should never happen.
> + */
> + if (KVM_BUG_ON(!spi.vcpu, kvm))
> + break;
> +
> /*
> * The page is good, but is stale. kvm_sync_page does
> * get the latest guest state, but (unlike mmu_unsync_children)
> @@ -2053,7 +2065,7 @@ static struct kvm_mmu_page *kvm_mmu_find_shadow_page(struct kvm *kvm,
> * If the sync fails, the page is zapped. If so, break
> * in order to rebuild it.
> */
> - ret = kvm_sync_page(vcpu, sp, &invalid_list);
> + ret = kvm_sync_page(spi.vcpu, sp, &invalid_list);
> if (ret < 0)
> break;
>
> @@ -2120,7 +2132,7 @@ static struct kvm_mmu_page *kvm_mmu_alloc_shadow_page(struct kvm *kvm,
> }
>
> static struct kvm_mmu_page *__kvm_mmu_get_shadow_page(struct kvm *kvm,
> - struct kvm_vcpu *vcpu,
> + struct sync_page_info spi,
> struct shadow_page_caches *caches,
> gfn_t gfn,
> union kvm_mmu_page_role role)
> @@ -2131,7 +2143,7 @@ static struct kvm_mmu_page *__kvm_mmu_get_shadow_page(struct kvm *kvm,
>
> sp_list = &kvm->arch.mmu_page_hash[kvm_page_table_hashfn(gfn)];
>
> - sp = kvm_mmu_find_shadow_page(kvm, vcpu, gfn, sp_list, role);
> + sp = kvm_mmu_find_shadow_page(kvm, spi, gfn, sp_list, role);
> if (!sp) {
> created = true;
> sp = kvm_mmu_alloc_shadow_page(kvm, caches, gfn, sp_list, role);
> @@ -2151,7 +2163,11 @@ static struct kvm_mmu_page *kvm_mmu_get_shadow_page(struct kvm_vcpu *vcpu,
> .gfn_array_cache = &vcpu->arch.mmu_gfn_array_cache,
> };
>
> - return __kvm_mmu_get_shadow_page(vcpu->kvm, vcpu, &caches, gfn, role);
> + struct sync_page_info spi = {
> + .vcpu = vcpu,
> + };
> +
> + return __kvm_mmu_get_shadow_page(vcpu->kvm, spi, &caches, gfn, role);
> }
>
> static union kvm_mmu_page_role kvm_mmu_child_role(u64 *sptep, bool direct, u32 access)
>
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