Re: [Qemu-devel] [PATCH 1/4] hw/i386/pc: hold the BQL when calling cpu_get_ticks

From: Paolo Bonzini <pbonzini@redhat.com>
To: "Emilio G. Cota" <cota@braap.org>, qemu-devel@nongnu.org
Cc: "Peter Crosthwaite" <crosthwaite.peter@gmail.com>,
	"Richard Henderson" <rth@twiddle.net>,
	"Eduardo Habkost" <ehabkost@redhat.com>,
	"Michael S . Tsirkin" <mst@redhat.com>,
	"Marcel Apfelbaum" <marcel.apfelbaum@gmail.com>,
	"Alex Bennée" <alex.bennee@linaro.org>
Subject: Re: [Qemu-devel] [PATCH 1/4] hw/i386/pc: hold the BQL when calling cpu_get_ticks
Date: Tue, 14 Aug 2018 08:37:49 +0200	[thread overview]
Message-ID: <54ac7b1f-7430-453f-cbba-b7fef74d3187@redhat.com> (raw)
In-Reply-To: <20180814013801.26036-2-cota@braap.org>

On 14/08/2018 03:37, Emilio G. Cota wrote:
> Signed-off-by: Emilio G. Cota <cota@braap.org>
> ---
>  hw/i386/pc.c | 13 ++++++++++++-
>  1 file changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/i386/pc.c b/hw/i386/pc.c
> index 83a444472b..7371cd9960 100644
> --- a/hw/i386/pc.c
> +++ b/hw/i386/pc.c
> @@ -158,7 +158,18 @@ static uint64_t ioportF0_read(void *opaque, hwaddr addr, unsigned size)
>  /* TSC handling */
>  uint64_t cpu_get_tsc(CPUX86State *env)
>  {
> -    return cpu_get_ticks();
> +    uint64_t ret;
> +    bool locked;
> +
> +    locked = qemu_mutex_iothread_locked();
> +    if (!locked) {
> +        qemu_mutex_lock_iothread();
> +    }
> +    ret = cpu_get_ticks();
> +    if (!locked) {
> +        qemu_mutex_unlock_iothread();
> +    }
> +    return ret;
>  }
>  
>  /* IRQ handling */
> 

We could use a spinlock for the rare case of timers_state.cpu_ticks_prev
> ticks (or even, on 64-bit, a seqlock+spinlock).  I'll give it a shot.

Paolo