Re: [PATCH v3 09/70] physmem: Introduce ram_block_convert_range() for page conversion

[David Hildenbrand <david@xxxxxxxxxx>]

On 08.12.23 08:59, Xiaoyao Li wrote:
> On 11/18/2023 5:03 AM, Isaku Yamahata wrote:
> > On Wed, Nov 15, 2023 at 02:14:18AM -0500,
> > Xiaoyao Li <xiaoyao.li@xxxxxxxxx> wrote:
> >
> > > It's used for discarding opposite memory after memory conversion, for
> > > confidential guest.
> > >
> > > When page is converted from shared to private, the original shared
> > > memory can be discarded via ram_block_discard_range();
> > >
> > > When page is converted from private to shared, the original private
> > > memory is back'ed by guest_memfd. Introduce
> > > ram_block_discard_guest_memfd_range() for discarding memory in
> > > guest_memfd.
> > >
> > > Originally-from: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
> > > Codeveloped-by: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>
> > > Signed-off-by: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>
> > > ---
> > >    include/exec/cpu-common.h |  2 ++
> > >    system/physmem.c          | 50 +++++++++++++++++++++++++++++++++++++++
> > >    2 files changed, 52 insertions(+)
> > >
> > > diff --git a/include/exec/cpu-common.h b/include/exec/cpu-common.h
> > > index 41115d891940..de728a18eef2 100644
> > > --- a/include/exec/cpu-common.h
> > > +++ b/include/exec/cpu-common.h
> > > @@ -175,6 +175,8 @@ typedef int (RAMBlockIterFunc)(RAMBlock *rb, void *opaque);
> > >    
> > >    int qemu_ram_foreach_block(RAMBlockIterFunc func, void *opaque);
> > >    int ram_block_discard_range(RAMBlock *rb, uint64_t start, size_t length);
> > > +int ram_block_convert_range(RAMBlock *rb, uint64_t start, size_t length,
> > > +                            bool shared_to_private);
> > >    
> > >    #endif
> > >    
> > > diff --git a/system/physmem.c b/system/physmem.c
> > > index ddfecddefcd6..cd6008fa09ad 100644
> > > --- a/system/physmem.c
> > > +++ b/system/physmem.c
> > > @@ -3641,6 +3641,29 @@ err:
> > >        return ret;
> > >    }
> > >    
> > > +static int ram_block_discard_guest_memfd_range(RAMBlock *rb, uint64_t start,
> > > +                                               size_t length)
> > > +{
> > > +    int ret = -1;
> > > +
> > > +#ifdef CONFIG_FALLOCATE_PUNCH_HOLE
> > > +    ret = fallocate(rb->guest_memfd, FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,
> > > +                    start, length);
> > > +
> > > +    if (ret) {
> > > +        ret = -errno;
> > > +        error_report("%s: Failed to fallocate %s:%" PRIx64 " +%zx (%d)",
> > > +                     __func__, rb->idstr, start, length, ret);
> > > +    }
> > > +#else
> > > +    ret = -ENOSYS;
> > > +    error_report("%s: fallocate not available %s:%" PRIx64 " +%zx (%d)",
> > > +                 __func__, rb->idstr, start, length, ret);
> > > +#endif
> > > +
> > > +    return ret;
> > > +}
> > > +
> > >    bool ramblock_is_pmem(RAMBlock *rb)
> > >    {
> > >        return rb->flags & RAM_PMEM;
> > > @@ -3828,3 +3851,30 @@ bool ram_block_discard_is_required(void)
> > >        return qatomic_read(&ram_block_discard_required_cnt) ||
> > >               qatomic_read(&ram_block_coordinated_discard_required_cnt);
> > >    }
> > > +
> > > +int ram_block_convert_range(RAMBlock *rb, uint64_t start, size_t length,
> > > +                            bool shared_to_private)
> > > +{
> > > +    if (!rb || rb->guest_memfd < 0) {
> > > +        return -1;
> > > +    }
> > > +
> > > +    if (!QEMU_PTR_IS_ALIGNED(start, qemu_host_page_size) ||
> > > +        !QEMU_PTR_IS_ALIGNED(length, qemu_host_page_size)) {
> > > +        return -1;
> > > +    }
> > > +
> > > +    if (!length) {
> > > +        return -1;
> > > +    }
> > > +
> > > +    if (start + length > rb->max_length) {
> > > +        return -1;
> > > +    }
> > > +
> > > +    if (shared_to_private) {
> > > +        return ram_block_discard_range(rb, start, length);
> > > +    } else {
> > > +        return ram_block_discard_guest_memfd_range(rb, start, length);
> > > +    }
> > > +}
> >
> > Originally this function issued KVM_SET_MEMORY_ATTRIBUTES, the function name
> > mad sense. But now it doesn't, and it issues only punch hole. We should rename
> > it to represent what it actually does. discard_range?
>
> ram_block_discard_range() already exists for non-guest-memfd memory discard.
>
> I cannot come up with a proper name. e.g.,
> ram_block_discard_opposite_range() while *opposite* seems unclear.
>
> Do you have any better idea?

Having some indication that this is about "guest_memfd" back and forth 
switching/conversion will make sense. But I'm also not able to come up 
with a better name.

Maybe have two functions:

ram_block_activate_guest_memfd_range
ram_block_deactivate_guest_memfd_range

--
Cheers,

David / dhildenb