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Re: [Qemu-devel] [PATCH 1/4] cpus: protect all icount computation with s

From: Emilio G. Cota
Subject: Re: [Qemu-devel] [PATCH 1/4] cpus: protect all icount computation with seqlock
Date: Fri, 31 Aug 2018 18:03:08 -0400
User-agent: Mutt/1.9.4 (2018-02-28)

On Mon, Aug 20, 2018 at 17:09:00 +0200, Paolo Bonzini wrote:
> Using the seqlock makes the atomic_read__nocheck safe, because it now
> happens always inside a seqlock and any torn reads will be retried.

Using a seqlock makes regular accesses safe as well, for the same
reason. It's undefined behaviour but I don't see how to avoid
it if the host might not have wide-enough atomics (see below).

> qemu_icount_bias and icount_time_shift also need to be accessed with
> atomics.

But qemu_icount_bias is always accessed through the seqlock, so regular
accesses should be fine there.

Moreover, seqlock + regular accesses allow us not to worry about
32-bit hosts without __atomic builtins, which might choke on
atomic accesses to u64's (regardless of __nocheck) like this one:

> -#ifdef CONFIG_ATOMIC64
> +    /* The read is protected by the seqlock, so __nocheck is okay.  */
>      return atomic_read__nocheck(&timers_state.qemu_icount);
> -#else /* FIXME: we need 64bit atomics to do this safely */
> -    return timers_state.qemu_icount;
> -#endif

So I think we should convert these to regular accesses. I just
wrote a patch to perform the conversion (after noticing the same
misuse of __nocheck + seqlock in qsp.c, which is my fault); however,
I have a question on patch 3, which I'd like to address first.



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