Re: [PATCH RFC 14/21] migration: Map hugetlbfs ramblocks twice, and pre-allocate

[Juan Quintela]

Peter Xu <peterx@redhat.com> wrote:
> On Mon, Jan 30, 2023 at 06:24:20AM +0100, Juan Quintela wrote:
>> I would consider here:
>> 
>>     uint8_t *host_doublemap;
>> 
>> as I have not a small name that means
>>     uint8_t *host_map_smaller_size_pages;
>> 
>> That explains why we need it.
>
> Sure, I can rename this one if it helps.
>
> One thing worth mention is that, it's not mapping things in small page size
> here with host_doublemap but in huge page size only.

Thanks.


> It's just that UFFDIO_CONTINUE needs another mapping to resolve the page
> faults. It'll be the guest hugetlb ramblocks that will be mapped in small
> pages during postcopy.

ok
>> Not initialized variables, remove the last two.
>
> I can do this.
>
>> 
>> > +    if (!migrate_hugetlb_doublemap()) {
>> > +        return 0;
>> > +    }
>> > +
>> 
>> I would move the declaration of the RAMBlock here.
>
> But isn't QEMU in most cases declaring variables at the start of any code
> block, rather than after or in the middle of any code segments?  IIRC some
> compiler should start to fail with it, even though not on the modern ones.

We can declare variables since c99.  Only 24 years have passed O:-)

Anyways:

Exhibit A: We already have that kind of code

static int nocomp_send_prepare(MultiFDSendParams *p, Error **errp)
{
    MultiFDPages_t *pages = p->pages;

    for (int i = 0; i < p->normal_num; i++) {
        p->iov[p->iovs_num].iov_base = pages->block->host + p->normal[i];
        p->iov[p->iovs_num].iov_len = p->page_size;
        p->iovs_num++;
    }

    p->next_packet_size = p->normal_num * p->page_size;
    p->flags |= MULTIFD_FLAG_NOCOMP;
    return nocomp;
}

Exhibit B:

from configure:

#if defined(__clang_major__) && defined(__clang_minor__)
# ifdef __apple_build_version__
#  if __clang_major__ < 10 || (__clang_major__ == 10 && __clang_minor__ < 0)
#   error You need at least XCode Clang v10.0 to compile QEMU
#  endif
# else
#  if __clang_major__ < 6 || (__clang_major__ == 6 && __clang_minor__ < 0)
#   error You need at least Clang v6.0 to compile QEMU
#  endif
# endif
#elif defined(__GNUC__) && defined(__GNUC_MINOR__)
# if __GNUC__ < 7 || (__GNUC__ == 7 && __GNUC_MINOR__ < 4)
#  error You need at least GCC v7.4.0 to compile QEMU
# endif
#else
# error You either need GCC or Clang to compiler QEMU
#endif
int main (void) { return 0; }
EOF

gcc-7.4.0: supports C11, so we are good here
https://gcc.gnu.org/onlinedocs/gcc-7.4.0/gcc/Standards.html#C-Language

clang 6.0: supports c11 and c17 standard
https://releases.llvm.org/6.0.0/tools/clang/docs/ReleaseNotes.html


So as far as I can see, we are good here.

Later, Juan.