Re: [PATCH v4 7/9] migration: Simplify alignment and alignment checks

[Peter Xu]

On Fri, Sep 03, 2021 at 12:07:20PM +0200, David Hildenbrand wrote:
> On 03.09.21 10:47, David Hildenbrand wrote:
> > On 03.09.21 00:32, Peter Xu wrote:
> > > On Thu, Sep 02, 2021 at 03:14:30PM +0200, David Hildenbrand wrote:
> > > > diff --git a/migration/migration.c b/migration/migration.c
> > > > index bb909781b7..ae97c2c461 100644
> > > > --- a/migration/migration.c
> > > > +++ b/migration/migration.c
> > > > @@ -391,7 +391,7 @@ int 
> > > > migrate_send_rp_message_req_pages(MigrationIncomingState *mis,
> > > >    int migrate_send_rp_req_pages(MigrationIncomingState *mis,
> > > >                                  RAMBlock *rb, ram_addr_t start, 
> > > > uint64_t haddr)
> > > >    {
> > > > -    void *aligned = (void *)(uintptr_t)(haddr & 
> > > > (-qemu_ram_pagesize(rb)));
> > > > +    void *aligned = (void *)QEMU_ALIGN_DOWN(haddr, 
> > > > qemu_ram_pagesize(rb));
> > > 
> > > Is uintptr_t still needed?  I thought it would generate a warning 
> > > otherwise but
> > > not sure.
> > 
> > It doesn't in my setup, but maybe it will on 32bit archs ...
> > 
> > I discussed this with Phil in
> > 
> > https://lkml.kernel.org/r/2c8d80ad-f171-7d5f-3235-92f02fa174b3@redhat.com
> > 
> > Maybe
> > 
> > QEMU_ALIGN_PTR_DOWN((void *)haddr, qemu_ram_pagesize(rb)));
> > 
> > Is really what we want.
> 
> ... but it would suffer the same issue I think. I just ran it trough the
> gitlab pipeline, including "i386-fedora-cross-compile" ... and it seems to
> compile just fine, which is weird, because I'd also expect
> 
> "warning: cast to pointer from integer of different size
> [-Wint-to-pointer-cast]"
> 
> We most certainly need the "(void *)(uintptr_t)" to convert from u64 to a
> pointer.
> 
> Let's just do it cleanly:
> 
> void *unaligned = (void *)(uintptr_t)haddr;
> void *aligned = QEMU_ALIGN_PTR_DOWN(unaligned, qemu_ram_pagesize(rb));
> 
> Thoughts?

---8<---
$ cat a.c
#include <stdio.h>
#include <time.h>
#include <assert.h>

#define ROUND_DOWN(n, d) ((n) & -(0 ? (n) : (d)))
#define QEMU_ALIGN_DOWN(n, m) ((n) / (m) * (m))

unsigned long getns(void)
{
    struct timespec tp;

    clock_gettime(CLOCK_MONOTONIC, &tp);
    return tp.tv_sec * 1000000000 + tp.tv_nsec;
}

void main(void)
{
    int i;
    unsigned long start, end, v1 = 0x1234567890, v2 = 0x1000;

    start = getns();
    for (i = 0; i < 1000000; i++) {
        v1 = ROUND_DOWN(v1, v2);
    }
    end = getns();
    printf("ROUND_DOWN took: \t%ld (us)\n", (end - start) / 1000);

    start = getns();
    for (i = 0; i < 1000000; i++) {
        v1 = QEMU_ALIGN_DOWN(v1, v2);
    }
    end = getns();
    printf("QEMU_ALIGN_DOWN took: \t%ld (us)\n", (end - start) / 1000);
}
$ make a
$ ./a
ROUND_DOWN took:        1445 (us)
QEMU_ALIGN_DOWN took:   9684 (us)
---8<---

So it's ~5 times slower here on the laptop, even if not very stable.  Agree
it's not a big deal. :)

It's just that since we know it's still faster, I then second:

  (uinptr_t)ROUND_DOWN(...);

Thanks,

-- 
Peter Xu