Re: [PATCH] Revert "ALSA: memalloc: Workaround for Xen PV"

[Takashi Iwai <tiwai@xxxxxxx>]

On Fri, 06 Sep 2024 20:42:09 +0200,
Ariadne Conill wrote:
> 
> This patch attempted to work around a DMA issue involving Xen, but
> causes subtle kernel memory corruption.
> 
> When I brought up this patch in the XenDevel matrix channel, I was
> told that it had been requested by the Qubes OS developers because
> they were trying to fix an issue where the sound stack would fail
> after a few hours of uptime.  They wound up disabling SG buffering
> entirely instead as a workaround.
> 
> Accordingly, I propose that we should revert this workaround patch,
> since it causes kernel memory corruption and that the ALSA and Xen
> communities should collaborate on fixing the underlying problem in
> such a way that SG buffering works correctly under Xen.
> 
> This reverts commit 53466ebdec614f915c691809b0861acecb941e30.
> 
> Signed-off-by: Ariadne Conill <ariadne@ariadne.space>
> Cc: stable@xxxxxxxxxxxxxxx
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx
> Cc: alsa-devel@xxxxxxxxxxxxxxxx
> Cc: Takashi Iwai <tiwai@xxxxxxx>

The relevant code has been largely rewritten for 6.12, so please check
the behavior with sound.git tree for-next branch.  I guess the same
issue should happen as the Xen workaround was kept and applied there,
too, but it has to be checked at first.

If the issue is persistent with there, the fix for 6.12 code would be
rather much simpler like the blow.


thanks,

Takashi

--- a/sound/core/memalloc.c
+++ b/sound/core/memalloc.c
@@ -793,9 +793,6 @@ static void *snd_dma_sg_alloc(struct snd_dma_buffer *dmab, 
size_t size)
        int type = dmab->dev.type;
        void *p;
 
-       if (cpu_feature_enabled(X86_FEATURE_XENPV))
-               return snd_dma_sg_fallback_alloc(dmab, size);
-
        /* try the standard DMA API allocation at first */
        if (type == SNDRV_DMA_TYPE_DEV_WC_SG)
                dmab->dev.type = SNDRV_DMA_TYPE_DEV_WC;