Re: [PATCH v3 16/23] KVM: x86/mmu: Cache the access bits of shadowed translations

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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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