Re: [RFC PATCH 39/39] KVM: guest_memfd: Dynamically split/reconstruct HugeTLB page

Ackerley Tng
2025-04-25T22:45:22Z

Yan Zhao <yan.y.zhao@intel.com> writes:

> On Thu, Apr 24, 2025 at 11:15:11AM -0700, Ackerley Tng wrote:
>> Vishal Annapurve <vannapurve@google.com> writes:
>>
>> > On Thu, Apr 24, 2025 at 1:15 AM Yan Zhao <yan.y.zhao@intel.com> wrote:
>> >>
>> >> On Thu, Apr 24, 2025 at 01:55:51PM +0800, Chenyi Qiang wrote:
>> >> >
>> >> >
>> >> > On 4/24/2025 12:25 PM, Yan Zhao wrote:
>> >> > > On Thu, Apr 24, 2025 at 09:09:22AM +0800, Yan Zhao wrote:
>> >> > >> On Wed, Apr 23, 2025 at 03:02:02PM -0700, Ackerley Tng wrote:
>> >> > >>> Yan Zhao <yan.y.zhao@intel.com> writes:
>> >> > >>>
>> >> > >>>> On Tue, Sep 10, 2024 at 11:44:10PM +0000, Ackerley Tng wrote:
>> >> > >>>>> +/*
>> >> > >>>>> + * Allocates and then caches a folio in the filemap. Returns a folio with
>> >> > >>>>> + * refcount of 2: 1 after allocation, and 1 taken by the filemap.
>> >> > >>>>> + */
>> >> > >>>>> +static struct folio *kvm_gmem_hugetlb_alloc_and_cache_folio(struct inode *inode,
>> >> > >>>>> + pgoff_t index)
>> >> > >>>>> +{
>> >> > >>>>> + struct kvm_gmem_hugetlb *hgmem;
>> >> > >>>>> + pgoff_t aligned_index;
>> >> > >>>>> + struct folio *folio;
>> >> > >>>>> + int nr_pages;
>> >> > >>>>> + int ret;
>> >> > >>>>> +
>> >> > >>>>> + hgmem = kvm_gmem_hgmem(inode);
>> >> > >>>>> + folio = kvm_gmem_hugetlb_alloc_folio(hgmem->h, hgmem->spool);
>> >> > >>>>> + if (IS_ERR(folio))
>> >> > >>>>> + return folio;
>> >> > >>>>> +
>> >> > >>>>> + nr_pages = 1UL << huge_page_order(hgmem->h);
>> >> > >>>>> + aligned_index = round_down(index, nr_pages);
>> >> > >>>> Maybe a gap here.
>> >> > >>>>
>> >> > >>>> When a guest_memfd is bound to a slot where slot->base_gfn is not aligned to
>> >> > >>>> 2M/1G and slot->gmem.pgoff is 0, even if an index is 2M/1G aligned, the
>> >> > >>>> corresponding GFN is not 2M/1G aligned.
>> >> > >>>
>> >> > >>> Thanks for looking into this.
>> >> > >>>
>> >> > >>> In 1G page support for guest_memfd, the offset and size are always
>> >> > >>> hugepage aligned to the hugepage size requested at guest_memfd creation
>> >> > >>> time, and it is true that when binding to a memslot, slot->base_gfn and
>> >> > >>> slot->npages may not be hugepage aligned.
>> >> > >>>
>> >> > >>>>
>> >> > >>>> However, TDX requires that private huge pages be 2M aligned in GFN.
>> >> > >>>>
>> >> > >>>
>> >> > >>> IIUC other factors also contribute to determining the mapping level in
>> >> > >>> the guest page tables, like lpage_info and .private_max_mapping_level()
>> >> > >>> in kvm_x86_ops.
>> >> > >>>
>> >> > >>> If slot->base_gfn and slot->npages are not hugepage aligned, lpage_info
>> >> > >>> will track that and not allow faulting into guest page tables at higher
>> >> > >>> granularity.
>> >> > >>
>> >> > >> lpage_info only checks the alignments of slot->base_gfn and
>> >> > >> slot->base_gfn + npages. e.g.,
>> >> > >>
>> >> > >> if slot->base_gfn is 8K, npages is 8M, then for this slot,
>> >> > >> lpage_info[2M][0].disallow_lpage = 1, which is for GFN [4K, 2M+8K);
>> >> > >> lpage_info[2M][1].disallow_lpage = 0, which is for GFN [2M+8K, 4M+8K);
>> >> > >> lpage_info[2M][2].disallow_lpage = 0, which is for GFN [4M+8K, 6M+8K);
>> >> > >> lpage_info[2M][3].disallow_lpage = 1, which is for GFN [6M+8K, 8M+8K);
>> >> >
>> >> > Should it be?
>> >> > lpage_info[2M][0].disallow_lpage = 1, which is for GFN [8K, 2M);
>> >> > lpage_info[2M][1].disallow_lpage = 0, which is for GFN [2M, 4M);
>> >> > lpage_info[2M][2].disallow_lpage = 0, which is for GFN [4M, 6M);
>> >> > lpage_info[2M][3].disallow_lpage = 0, which is for GFN [6M, 8M);
>> >> > lpage_info[2M][4].disallow_lpage = 1, which is for GFN [8M, 8M+8K);
>> >> Right. Good catch. Thanks!
>> >>
>> >> Let me update the example as below:
>> >> slot->base_gfn is 2 (for GPA 8KB), npages 2000 (for a 8MB range)
>> >>
>> >> lpage_info[2M][0].disallow_lpage = 1, which is for GPA [8KB, 2MB);
>> >> lpage_info[2M][1].disallow_lpage = 0, which is for GPA [2MB, 4MB);
>> >> lpage_info[2M][2].disallow_lpage = 0, which is for GPA [4MB, 6MB);
>> >> lpage_info[2M][3].disallow_lpage = 0, which is for GPA [6MB, 8MB);
>> >> lpage_info[2M][4].disallow_lpage = 1, which is for GPA [8MB, 8MB+8KB);
>> >>
>> >> lpage_info indicates that a 2MB mapping is alllowed to cover GPA 4MB and GPA
>> >> 4MB+16KB. However, their aligned_index values lead guest_memfd to allocate two
>> >> 2MB folios, whose physical addresses may not be contiguous.
>> >>
>> >> Additionally, if the guest accesses two GPAs, e.g., GPA 2MB+8KB and GPA 4MB,
>> >> KVM could create two 2MB mappings to cover GPA ranges [2MB, 4MB), [4MB, 6MB).
>> >> However, guest_memfd just allocates the same 2MB folio for both faults.
>> >>
>> >>
>> >> >
>> >> > >>
>> >> > >> ---------------------------------------------------------
>> >> > >> | | | | | | | | |
>> >> > >> 8K 2M 2M+8K 4M 4M+8K 6M 6M+8K 8M 8M+8K
>> >> > >>
>> >> > >> For GFN 6M and GFN 6M+4K, as they both belong to lpage_info[2M][2], huge
>> >> > >> page is allowed. Also, they have the same aligned_index 2 in guest_memfd.
>> >> > >> So, guest_memfd allocates the same huge folio of 2M order for them.
>> >> > > Sorry, sent too fast this morning. The example is not right. The correct
>> >> > > one is:
>> >> > >
>> >> > > For GFN 4M and GFN 4M+16K, lpage_info indicates that 2M is allowed. So,
>> >> > > KVM will create a 2M mapping for them.
>> >> > >
>> >> > > However, in guest_memfd, GFN 4M and GFN 4M+16K do not correspond to the
>> >> > > same 2M folio and physical addresses may not be contiguous.
>> >
>> > Then during binding, guest memfd offset misalignment with hugepage
>> > should be same as gfn misalignment. i.e.
>> >
>> > (offset & ~huge_page_mask(h)) == ((slot->base_gfn << PAGE_SHIFT) &
>> > ~huge_page_mask(h));
>> >
>> > For non guest_memfd backed scenarios, KVM allows slot gfn ranges that
>> > are not hugepage aligned, so guest_memfd should also be able to
>> > support non-hugepage aligned memslots.
>> >
>>
>> I drew up a picture [1] which hopefully clarifies this.
>>
>> Thanks for pointing this out, I understand better now and we will add an
>> extra constraint during memslot binding of guest_memfd to check that gfn
>> offsets within a hugepage must be guest_memfd offsets.
> I'm a bit confused.
>
> As "index = gfn - slot->base_gfn + slot->gmem.pgoff", do you mean you are going
> to force "slot->base_gfn == slot->gmem.pgoff" ?
>
> For some memory region, e.g., "pc.ram", it's divided into 2 parts:
> - one with offset 0, size 0x80000000(2G),
> positioned at GPA 0, which is below GPA 4G;
> - one with offset 0x80000000(2G), size 0x80000000(2G),
> positioned at GPA 0x100000000(4G), which is above GPA 4G.
>
> For the second part, its slot->base_gfn is 0x100000000, while slot->gmem.pgoff
> is 0x80000000.
>

Nope I don't mean to enforce that they are equal, we just need the
offsets within the page to be equal.

I edited Vishal's code snippet, perhaps it would help explain better:

page_size is the size of the hugepage, so in our example,

page_size = SZ_2M;
page_mask = ~(page_size - 1);
offset_within_page = slot->gmem.pgoff & page_mask;
gfn_within_page = (slot->base_gfn << PAGE_SHIFT) & page_mask;

We will enforce that

offset_within_page == gfn_within_page;

>> Adding checks at binding time will allow hugepage-unaligned offsets (to
>> be at parity with non-guest_memfd backing memory) but still fix this
>> issue.
>>
>> lpage_info will make sure that ranges near the bounds will be
>> fragmented, but the hugepages in the middle will still be mappable as
>> hugepages.
>>
>> [1] https://lpc.events/event/18/contributions/1764/attachments/1409/3706/binding-must-have-same-alignment.svg