Re: kern/60144: virtio(4) cache coherence issue

["Tetsuya Isaki via gnats" <gnats-admin%NetBSD.org@localhost>]

The following reply was made to PR kern/60144; it has been noted by GNATS.

From: Tetsuya Isaki <isaki%pastel-flower.jp@localhost>
To: Taylor R Campbell <riastradh%NetBSD.org@localhost>
Cc: Jason Thorpe <thorpej%NetBSD.org@localhost>,
	netbsd-bugs%NetBSD.org@localhost,
	gnats-bugs%NetBSD.org@localhost
Subject: Re: kern/60144: virtio(4) cache coherence issue
Date: Tue, 31 Mar 2026 19:34:03 +0900

 At Mon, 30 Mar 2026 03:47:22 +0000,
 Taylor R Campbell wrote:
 > > I think that vq_sync_uring_header(sc, vq, BUS_DMASYNC_PREREAD) is
 > > necessary to invalidate the cache line before reading fresh
 > > vq->vq_used->idx (at line 610) ?
 > 
 > I believe you have got the usage model of the preread vs postread
 > wrong -- it is confusing, because `PREREAD' does not mean `before we
 > read from the buffer' but rather `before the device does a DMA read
 > operation to initialize the buffer'.
 (snip)
 
 Thank you for explanation!
 It's confusing...
 
 > If some kind of cache invalidation must happen between the interrupt
 > delivery or DMA_STATUS_REG read at (3), and the use of the data in the
 > buffer at (5), then the m68k and virt68k bus_dmamap_sync logic might
 > be missing something for BUS_DMAMAP_POSTREAD -- it seems to have cases
 > only for BUS_DMAMAP_PREWRITE and BUS_DMAMAP_PREREAD:
 > 
 > https://nxr.NetBSD.org/xref/src/sys/arch/m68k/m68k/bus_dma.c?r=1.41#491
 > https://nxr.NetBSD.org/xref/src/sys/arch/virt68k/virt68k/bus_dma.c?r=1.4#476
 > 
 > Perhaps m68k needs new POSTREAD logic to be written, or perhaps there
 > was a mistake and the PREREAD logic should have been for POSTREAD?
 
 Hmm, that's it... Thanks!
 
 sys/arch/virt68k/virt68k/bus_dma.c:
   429 void
   430 _bus_dmamap_sync_030(bus_dma_tag_t t, bus_dmamap_t map, bus_addr_t offset,
   431     bus_size_t len, int ops)
   432 {
   433
   434     /* Nothing yet */
   435 }
 
 I try to implement (and test) it.
 Or would someone take this?
 ---
 Tetsuya Isaki <isaki%pastel-flower.jp@localhost / isaki%NetBSD.org@localhost>