Re: [PATCH 06/13] KVM: Disallow hugepages for incompatible gmem bindings, but let 'em succeed

[Sean Christopherson <seanjc@google.com>]

On Mon, Oct 02, 2023, Michael Roth wrote:
> On Thu, Sep 28, 2023 at 11:31:51AM -0700, Sean Christopherson wrote:
> > On Fri, Sep 22, 2023, Michael Roth wrote:
> > > On Thu, Sep 21, 2023 at 01:33:23PM -0700, Sean Christopherson wrote:
> > > > +	/*
> > > > +	 * For simplicity, allow mapping a hugepage if and only if the entire
> > > > +	 * binding is compatible, i.e. don't bother supporting mapping interior
> > > > +	 * sub-ranges with hugepages (unless userspace comes up with a *really*
> > > > +	 * strong use case for needing hugepages within unaligned bindings).
> > > > +	 */
> > > > +	if (!IS_ALIGNED(slot->gmem.pgoff, 1ull << *max_order) ||
> > > > +	    !IS_ALIGNED(slot->npages, 1ull << *max_order))
> > > > +		*max_order = 0;
> > > 
> > > Thanks for working this in. Unfortunately on x86 the bulk of guest memory
> > > ends up getting slotted directly above legacy regions at GFN 0x100, 
> > 
> > Can you provide an example?  I'm struggling to understand what the layout actually
> > is.  I don't think it changes the story for the kernel, but it sounds like there
> > might be room for optimization in QEMU?  Or more likely, I just don't understand
> > what you're saying :-)
> 
> Here's one example, which seems to be fairly normal for an x86 boot:
> 
>   kvm_set_user_memory AddrSpace#0 Slot#0 flags=0x4 gpa=0x0 size=0x80000000 ua=0x7f24afc00000 ret=0 restricted_fd=19 restricted_offset=0x0
>   ^ QEMU creates Slot 0 for all of main guest RAM
>   kvm_set_user_memory AddrSpace#0 Slot#0 flags=0x0 gpa=0x0 size=0x0 ua=0x7f24afc00000 ret=0 restricted_fd=19 restricted_offset=0x0
>   kvm_set_user_memory AddrSpace#0 Slot#0 flags=0x4 gpa=0x0 size=0xc0000 ua=0x7f24afc00000 ret=0 restricted_fd=19 restricted_offset=0x0
>   kvm_set_user_memory AddrSpace#0 Slot#3 flags=0x6 gpa=0xc0000 size=0x20000 ua=0x7f2575000000 ret=0 restricted_fd=33 restricted_offset=0x0
>   kvm_set_user_memory AddrSpace#0 Slot#4 flags=0x6 gpa=0xe0000 size=0x20000 ua=0x7f2575400000 ret=0 restricted_fd=31 restricted_offset=0x0
>   ^ legacy regions are created and mapped on top of GPA ranges [0xc0000:0xe0000) and [0xe0000:0x100000)
>   kvm_set_user_memory AddrSpace#0 Slot#5 flags=0x4 gpa=0x100000 size=0x7ff00000 ua=0x7f24afd00000 ret=0 restricted_fd=19 restricted_offset=0x100000
>   ^ QEMU divides Slot 0 into Slot 0 at [0x0:0xc0000) and Slot 5 at [0x100000:0x80000000)
>     Both Slots still share the same backing memory allocation, so same gmem
>     fd 19 is used,but Slot 5 is assigned to offset 0x100000, whih is not
>     2M-aligned
> 
> I tried messing with QEMU handling to pad out guest_memfd offsets to 2MB
> boundaries but then the inode size needs to be enlarged to account for it
> and things get a bit messy. Not sure if there are alternative approaches
> that can be taken from userspace, but with normal malloc()'d or mmap()'d
> backing memory the kernel can still allocate a 2MB backing page for the
> [0x0:0x200000) range and I think KVM still handles that when setting up
> NPT of sub-ranges so there might not be much room for further optimization
> there.

Oooh, duh.  QEMU intentionally creates a gap for the VGA and/or BIOS holes, and
so the lower DRAM chunk that goes from the end of the system reserved chunk to
to TOLUD is started at an unaligned offset, even though 99% of the slot is properly
aligned.

Yeah, KVM definitely needs to support that.  Requiring userspace to align based
on the hugepage size could work, e.g. QEMU could divide slot 5 into N slots, to
end up with a series of slots to get from 4KiB aligned => 2MiB aligned => 1GiB
aligned.  But pushing for that would be beyond stubborn.

Thanks for being patient :-)