On Fri, Apr 8, 2022 at 5:02 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> On Fri, Apr 01, 2022, David Matlack wrote:
> > @@ -733,7 +733,7 @@ static void mmu_free_pte_list_desc(struct pte_list_desc *pte_list_desc)
> > static gfn_t kvm_mmu_page_get_gfn(struct kvm_mmu_page *sp, int index)
> > {
> > if (!sp->role.direct)
> > - return sp->gfns[index];
> > + return sp->shadowed_translation[index].gfn;
> >
> > return sp->gfn + (index << ((sp->role.level - 1) * PT64_LEVEL_BITS));
> > }
> > @@ -741,7 +741,7 @@ static gfn_t kvm_mmu_page_get_gfn(struct kvm_mmu_page *sp, int index)
> > static void kvm_mmu_page_set_gfn(struct kvm_mmu_page *sp, int index, gfn_t gfn)
>
> This should be replaced with a single helper to set the gfn+access. Under no
> circumstance should _just_ the gfn change, and that will allow us to optimize
> writing the entry. More below.
>
> > {
> > if (!sp->role.direct) {
> > - sp->gfns[index] = gfn;
> > + sp->shadowed_translation[index].gfn = gfn;
> > return;
> > }
> >
> > @@ -752,6 +752,47 @@ static void kvm_mmu_page_set_gfn(struct kvm_mmu_page *sp, int index, gfn_t gfn)
> > kvm_mmu_page_get_gfn(sp, index), gfn);
> > }
> >
> > +static void kvm_mmu_page_set_access(struct kvm_mmu_page *sp, int index, u32 access)
> > +{
> > + if (!sp->role.direct) {
> > + sp->shadowed_translation[index].access = access;
> > + return;
> > + }
> > +
> > + if (WARN_ON(access != sp->role.access))
> > + pr_err_ratelimited("access mismatch under direct page %llx "
>
> LOL, I realize this is not your code, but ratelimiting under a WARN ain't gonna
> help much :-)
Ha! Yeah this silly. I'll see about adding a precursor patch to make
it less terrible.
>
> This also generates a warning and fails to compile with KVM_WERROR=y, though I
> believe the test bots already reported that.
>
>
> arch/x86/kvm/mmu/mmu.c: In function ‘kvm_mmu_page_set_access’:
> include/linux/kern_levels.h:5:25: error: format ‘%llx’ expects argument of type ‘long long unsigned int’, but argument 3 has type ‘int’ [-Werror=format=]
> 5 | #define KERN_SOH "\001" /* ASCII Start Of Header */
> | ^~~~~~
> include/linux/printk.h:418:25: note: in definition of macro ‘printk_index_wrap’
> 418 | _p_func(_fmt, ##__VA_ARGS__); \
> | ^~~~
> include/linux/printk.h:640:17: note: in expansion of macro ‘printk’
> 640 | printk(fmt, ##__VA_ARGS__); \
> | ^~~~~~
> include/linux/printk.h:654:9: note: in expansion of macro ‘printk_ratelimited’
> 654 | printk_ratelimited(KERN_ERR pr_fmt(fmt), ##__VA_ARGS__)
> | ^~~~~~~~~~~~~~~~~~
> include/linux/kern_levels.h:11:25: note: in expansion of macro ‘KERN_SOH’
> 11 | #define KERN_ERR KERN_SOH "3" /* error conditions */
> | ^~~~~~~~
> include/linux/printk.h:654:28: note: in expansion of macro ‘KERN_ERR’
> 654 | printk_ratelimited(KERN_ERR pr_fmt(fmt), ##__VA_ARGS__)
> | ^~~~~~~~
> arch/x86/kvm/mmu/mmu.c:763:17: note: in expansion of macro ‘pr_err_ratelimited’
> 763 | pr_err_ratelimited("access mismatch under direct page %llx "
> | ^~~~~~~~~~~~~~~~~~
>
>
> > + "(expected %llx, got %llx)\n",
> > + kvm_mmu_page_get_gfn(sp, index),
> > + sp->role.access, access);
> > +}
> > +
> > +/*
> > + * For leaf SPTEs, fetch the *guest* access permissions being shadowed. Note
> > + * that the SPTE itself may have a more constrained access permissions that
> > + * what the guest enforces. For example, a guest may create an executable
> > + * huge PTE but KVM may disallow execution to mitigate iTLB multihit.
> > + */
> > +static u32 kvm_mmu_page_get_access(struct kvm_mmu_page *sp, int index)
> > +{
> > + if (!sp->role.direct)
> > + return sp->shadowed_translation[index].access;
> > +
> > + /*
> > + * For direct MMUs (e.g. TDP or non-paging guests) there are no *guest*
> > + * access permissions being shadowed. So we can just return ACC_ALL
> > + * here.
> > + *
> > + * For indirect MMUs (shadow paging), direct shadow pages exist when KVM
> > + * is shadowing a guest huge page with smaller pages, since the guest
> > + * huge page is being directly mapped. In this case the guest access
> > + * permissions being shadowed are the access permissions of the huge
> > + * page.
> > + *
> > + * In both cases, sp->role.access contains exactly what we want.
> > + */
> > + return sp->role.access;
> > +}
>
> ...
>
> > diff --git a/arch/x86/kvm/mmu/mmu_internal.h b/arch/x86/kvm/mmu/mmu_internal.h
> > index b6e22ba9c654..3f76f4c1ae59 100644
> > --- a/arch/x86/kvm/mmu/mmu_internal.h
> > +++ b/arch/x86/kvm/mmu/mmu_internal.h
> > @@ -32,6 +32,18 @@ extern bool dbg;
> >
> > typedef u64 __rcu *tdp_ptep_t;
> >
> > +/*
> > + * Stores the result of the guest translation being shadowed by an SPTE. KVM
> > + * shadows two types of guest translations: nGPA -> GPA (shadow EPT/NPT) and
> > + * GVA -> GPA (traditional shadow paging). In both cases the result of the
> > + * translation is a GPA and a set of access constraints.
> > + */
> > +struct shadowed_translation_entry {
> > + /* Note, GFNs can have at most 64 - PAGE_SHIFT = 52 bits. */
> > + u64 gfn:52;
> > + u64 access:3;
>
> A bitfield is completely unnecessary and generates bad code. As is, it generates
> _really_ bad code because extracting and setting requires non-standard 64-bit value
> masks, multiple operations, and accesses to unaligned data. The generated code can
> be made slightly less awful by using a fully byte for access and 64 bits for GFN,
> but it still sucks compared to what we can hand generate.
>
> The other aspect of this is that retrieving the GFN is a frequent operation,
> whereas the access is almost never read. I.e. we should bias for reading the GFN
> above all else.
>
> The simple and obvious thing is to not reinvent the wheel. GFN = (GPA >> PAGE_SHIFT),
> and ignoring NX, access lives in the lower 12 bits of a PTE. Then reading the GFN is
> a simple SHR, and reading access info is a simple AND.
>
> We might also be able to optimize FNAME(sync_page), but I don't care much about
> that, it's rarely used for nested TDP.
>
> So, keep translation_entry a gfn_t *, then do:
Looks good, will do in v4.
>
> static gfn_t kvm_mmu_page_get_gfn(struct kvm_mmu_page *sp, int index)
> {
> if (!sp->role.direct)
> return sp->shadowed_translation[index] >> PAGE_SHIFT;
>
> return sp->gfn + (index << ((sp->role.level - 1) * PT64_LEVEL_BITS));
> }
>
> static void kvm_mmu_page_set_translation(struct kvm_mmu_page *sp, int index,
> gfn_t gfn, unsigned int access)
> {
> if (!sp->role.direct) {
> sp->shadowed_translation[index] = (gfn << PAGE_SHIFT) | access;
> return;
> }
>
> if (WARN_ON(gfn != kvm_mmu_page_get_gfn(sp, index)))
> pr_err_ratelimited("gfn mismatch under direct page %llx "
> "(expected %llx, got %llx)\n",
> sp->gfn,
> kvm_mmu_page_get_gfn(sp, index), gfn);
> }
>
> static void kvm_mmu_page_set_access(struct kvm_mmu_page *sp, int index,
> unsigned int access)
> {
> if (sp->role.direct)
> return;
>
> sp->shadowed_translation[index] &= PAGE_MASK;
> sp->shadowed_translation[index] |= access;
> }
>
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