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Re: [PATCH v6 6/8] xen/spinlock: support higher number of cpus
- To: Juergen Gross <jgross@xxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Tue, 2 Apr 2024 16:42:13 +0200
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, George Dunlap <george.dunlap@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Tue, 02 Apr 2024 14:42:23 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 27.03.2024 16:22, Juergen Gross wrote:
> Allow 16 bits per cpu number, which is the limit imposed by
> spinlock_tickets_t.
>
> This will allow up to 65535 cpus, while increasing only the size of
> recursive spinlocks in debug builds from 8 to 12 bytes.
>
> The current Xen limit of 4095 cpus is imposed by SPINLOCK_CPU_BITS
> being 12. There are machines available with more cpus than the current
> Xen limit, so it makes sense to have the possibility to use more cpus.
>
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
albeit I have to say that I'm not entirely convinced of ...
> --- a/xen/common/spinlock.c
> +++ b/xen/common/spinlock.c
> @@ -485,7 +485,9 @@ bool _rspin_trylock(rspinlock_t *lock)
>
> /* Don't allow overflow of recurse_cpu field. */
> BUILD_BUG_ON(NR_CPUS > SPINLOCK_NO_CPU);
> + BUILD_BUG_ON(SPINLOCK_CPU_BITS > sizeof(lock->recurse_cpu) * 8);
> BUILD_BUG_ON(SPINLOCK_RECURSE_BITS < 3);
> + BUILD_BUG_ON(SPINLOCK_MAX_RECURSE > ((1u << SPINLOCK_RECURSE_BITS) - 1));
>
> check_lock(&lock->debug, true);
... the two additions here: The two checks we had verify independent
properties, whereas the new ones basically check that struct rspinlock
and its associated #define-s were got right. We don't check such
elsewhere, I don't think.
Jan
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