linux-kernel - Re: [PATCH V5 5/7] KVM: selftests: add library for creating/interacting with SEV guests

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Date:   Mon, 9 Jan 2023 14:19:26 -0700
From:   Peter Gonda <pgonda@...gle.com>
To:     Ackerley Tng <ackerleytng@...gle.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        marcorr@...gle.com, seanjc@...gle.com, michael.roth@....com,
        thomas.lendacky@....com, joro@...tes.org, mizhang@...gle.com,
        pbonzini@...hat.com, andrew.jones@...ux.dev, vannapurve@...gle.com
Subject: Re: [PATCH V5 5/7] KVM: selftests: add library for
 creating/interacting with SEV guests

On Wed, Dec 21, 2022 at 2:13 PM Ackerley Tng <ackerleytng@...gle.com> wrote:
>
>
> > +static void encrypt_region(struct kvm_vm *vm, struct
> > userspace_mem_region *region)
> > +{
> > +     const struct sparsebit *protected_phy_pages =
> > +             region->protected_phy_pages;
> > +     const uint64_t memory_size = region->region.memory_size;
> > +     const vm_paddr_t gpa_start = region->region.guest_phys_addr;
> > +     sparsebit_idx_t pg = 0;
> > +
> > +     sev_register_user_region(vm, region);
> > +
> > +     while (pg < (memory_size / vm->page_size)) {
> > +             sparsebit_idx_t nr_pages;
> > +
> > +             if (sparsebit_is_clear(protected_phy_pages, pg)) {
> > +                     pg = sparsebit_next_set(protected_phy_pages, pg);
> > +                     if (!pg)
> > +                             break;
> > +             }
> > +
> > +             nr_pages = sparsebit_next_clear(protected_phy_pages, pg) - pg;
> > +             if (nr_pages <= 0)
> > +                     nr_pages = 1;
>
> I think this may not be correct in the case where the sparsebit has the
> range [x, 2**64-1] (inclusive) set. In that case, sparsebit_next_clear()
> will return 0, but the number of pages could be more than 1.
>
> > +
> > +             sev_launch_update_data(vm, gpa_start + pg * vm->page_size,
>
> Computing the beginning of the gpa range with
>
> gpa_start + pg * vm->page_size
>
> only works if this memory region's gpa_start is 0.
>
> > +                                    nr_pages * vm->page_size);
> > +             pg += nr_pages;
> > +     }
> > +}
>
> Here's a suggestion (I'm using this on a TDX version of this patch)

Thanks for this catch and the code. I've pulled this into the V6 I am preparing.

>
>
> /**
>   * Iterate over set ranges within sparsebit @s. In each iteration,
>   * @range_begin and @range_end will take the beginning and end of the set
> range,
>   * which are of type sparsebit_idx_t.
>   *
>   * For example, if the range [3, 7] (inclusive) is set, within the
> iteration,
>   * @range_begin will take the value 3 and @range_end will take the value 7.
>   *
>   * Ensure that there is at least one bit set before using this macro with
>   * sparsebit_any_set(), because sparsebit_first_set() will abort if none are
>   * set.
>   */
> #define sparsebit_for_each_set_range(s, range_begin, range_end)         \
>         for (range_begin = sparsebit_first_set(s),                      \
>                      range_end =                                        \
>                      sparsebit_next_clear(s, range_begin) - 1;          \
>              range_begin && range_end;                                  \
>              range_begin = sparsebit_next_set(s, range_end),            \
>                      range_end =                                        \
>                      sparsebit_next_clear(s, range_begin) - 1)
> /*
>   * sparsebit_next_clear() can return 0 if [x, 2**64-1] are all set, and the
> -1
>   * would then cause an underflow back to 2**64 - 1. This is expected and
>   * correct.
>   *
>   * If the last range in the sparsebit is [x, y] and we try to iterate,
>   * sparsebit_next_set() will return 0, and sparsebit_next_clear() will try
> and
>   * find the first range, but that's correct because the condition expression
>   * would cause us to quit the loop.
>   */
>
>
> static void encrypt_region(struct kvm_vm *vm, struct userspace_mem_region
> *region)
> {
>         const struct sparsebit *protected_phy_pages =
>                 region->protected_phy_pages;
>         const vm_paddr_t gpa_base = region->region.guest_phys_addr;
>         const sparsebit_idx_t lowest_page_in_region = gpa_base >> vm->page_shift;
>
>         sparsebit_idx_t i;
>         sparsebit_idx_t j;
>
>         if (!sparsebit_any_set(protected_phy_pages))
>                 return;
>
>         sev_register_user_region(vm, region);
>
>         sparsebit_for_each_set_range(protected_phy_pages, i, j) {
>                 const uint64_t size_to_load = (j - i + 1) * vm->page_size;
>                 const uint64_t offset = (i - lowest_page_in_region) * vm->page_size;
>                 const uint64_t gpa = gpa_base + offset;
>
>                 sev_launch_update_data(vm, gpa, size_to_load);
>         }
> }