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Re: [PATCH] pc-bios/s390-ccw: Use memory barriers in virtio code

From: Peter Maydell
Subject: Re: [PATCH] pc-bios/s390-ccw: Use memory barriers in virtio code
Date: Tue, 16 Feb 2021 16:44:48 +0000

On Tue, 16 Feb 2021 at 16:15, Thomas Huth <thuth@redhat.com> wrote:
> I was just about to reply that this is certainly not necessary, since
> the DIAGNOSE instruction that we use for the notification hypercall
> should be serializing anyway ... but after looking at the PoP, it
> actually is not marked as a serializing instruction! (while e.g.
> SVC - supervisor call - is explicitly marked as serializing)
> So maybe that's worth a try: Peter, could you please apply this patch
> on top an see whether it makes a difference?
> diff --git a/pc-bios/s390-ccw/virtio.c b/pc-bios/s390-ccw/virtio.c
> --- a/pc-bios/s390-ccw/virtio.c
> +++ b/pc-bios/s390-ccw/virtio.c
> @@ -54,6 +54,7 @@ static long kvm_hypercall(unsigned long nr, unsigned long 
> param1,
>       register ulong r_param3 asm("4") = param3;
>       register long retval asm("2");
> +    virtio_mb();
>       asm volatile ("diag 2,4,0x500"
>                     : "=d" (retval)
>                     : "d" (r_nr), "0" (r_param1), "r"(r_param2), "d"(r_param3)

Doesn't really help (maybe brings the occurrence rate down a bit
more, towards about 1 in 9?)

-- PMM

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