From: Mark Kettenis Subject: Re: virtio: Allow DMA mem > 4GB on amd64 To: Stefan Fritsch Cc: tech@openbsd.org Date: Thu, 31 Jul 2025 13:08:21 +0200 > Date: Thu, 31 Jul 2025 12:38:08 +0200 (CEST) > From: Stefan Fritsch > > Hi, > > the diff below allows to use mem above 4GB on amd64 for virtio rings and > descriptors. This may reduce pressure on dmaable memory a bit. > > For the virtio ring address, virtio 0.9 has a 2^44 limit, so I use > bus_dmamem_alloc_range there. > > ok? In principle yes, I think doing this makes sense. However... > diff --git a/sys/dev/pv/if_vio.c b/sys/dev/pv/if_vio.c > index f9c5d1ed0b4..1f46332d452 100644 > --- a/sys/dev/pv/if_vio.c > +++ b/sys/dev/pv/if_vio.c > @@ -392,11 +392,13 @@ vio_alloc_dmamem(struct vio_softc *sc) > int nsegs; > > if (bus_dmamap_create(vsc->sc_dmat, sc->sc_dma_size, 1, > - sc->sc_dma_size, 0, BUS_DMA_NOWAIT | BUS_DMA_ALLOCNOW, > + sc->sc_dma_size, 0, > + BUS_DMA_NOWAIT | BUS_DMA_ALLOCNOW | BUS_DMA_64BIT, > &sc->sc_dma_map) != 0) > goto err; > if (bus_dmamem_alloc(vsc->sc_dmat, sc->sc_dma_size, 16, 0, > - &sc->sc_dma_seg, 1, &nsegs, BUS_DMA_NOWAIT | BUS_DMA_ZERO) != 0) > + &sc->sc_dma_seg, 1, &nsegs, > + BUS_DMA_NOWAIT | BUS_DMA_ZERO | BUS_DMA_64BIT) != 0) > goto destroy; > if (bus_dmamem_map(vsc->sc_dmat, &sc->sc_dma_seg, nsegs, > sc->sc_dma_size, &sc->sc_dma_kva, BUS_DMA_NOWAIT) != 0) > @@ -542,7 +544,7 @@ vio_alloc_mem(struct vio_softc *sc, int tx_max_segments) > r = bus_dmamap_create(vsc->sc_dmat, > sc->sc_rx_mbuf_size + sc->sc_hdr_size, 2, > sc->sc_rx_mbuf_size, 0, > - BUS_DMA_NOWAIT|BUS_DMA_ALLOCNOW, > + BUS_DMA_NOWAIT | BUS_DMA_ALLOCNOW | BUS_DMA_64BIT, > &vioq->viq_rxdmamaps[i]); > if (r != 0) > goto destroy; > @@ -551,7 +553,7 @@ vio_alloc_mem(struct vio_softc *sc, int tx_max_segments) > for (i = 0; i < txqsize; i++) { > r = bus_dmamap_create(vsc->sc_dmat, txsize, > tx_max_segments, txsize, 0, > - BUS_DMA_NOWAIT|BUS_DMA_ALLOCNOW, > + BUS_DMA_NOWAIT | BUS_DMA_ALLOCNOW | BUS_DMA_64BIT, > &vioq->viq_txdmamaps[i]); > if (r != 0) > goto destroy; > diff --git a/sys/dev/pv/vioblk.c b/sys/dev/pv/vioblk.c > index c38efbac356..5d5c9f706d5 100644 > --- a/sys/dev/pv/vioblk.c > +++ b/sys/dev/pv/vioblk.c > @@ -646,7 +646,7 @@ vioblk_alloc_reqs(struct vioblk_softc *sc, int qsize) > > allocsize = sizeof(struct virtio_blk_req) * nreqs; > r = bus_dmamem_alloc(sc->sc_virtio->sc_dmat, allocsize, 0, 0, > - &sc->sc_reqs_segs[0], 1, &rsegs, BUS_DMA_NOWAIT); > + &sc->sc_reqs_segs[0], 1, &rsegs, BUS_DMA_NOWAIT | BUS_DMA_64BIT); > if (r != 0) { > printf("DMA memory allocation failed, size %d, error %d\n", > allocsize, r); > @@ -697,7 +697,8 @@ vioblk_alloc_reqs(struct vioblk_softc *sc, int qsize) > > r = bus_dmamap_create(sc->sc_virtio->sc_dmat, > VR_DMA_END, 1, VR_DMA_END, 0, > - BUS_DMA_NOWAIT|BUS_DMA_ALLOCNOW, &vr->vr_cmdsts); > + BUS_DMA_NOWAIT | BUS_DMA_ALLOCNOW | BUS_DMA_64BIT, > + &vr->vr_cmdsts); > if (r != 0) { > printf("cmd dmamap creation failed, err %d\n", r); > nreqs = i; > @@ -711,7 +712,8 @@ vioblk_alloc_reqs(struct vioblk_softc *sc, int qsize) > goto err_reqs; > } > r = bus_dmamap_create(sc->sc_virtio->sc_dmat, MAXPHYS, > - SEG_MAX, MAXPHYS, 0, BUS_DMA_NOWAIT|BUS_DMA_ALLOCNOW, > + SEG_MAX, MAXPHYS, 0, > + BUS_DMA_NOWAIT | BUS_DMA_ALLOCNOW | BUS_DMA_64BIT, > &vr->vr_payload); > if (r != 0) { > printf("payload dmamap creation failed, err %d\n", r); > diff --git a/sys/dev/pv/vioscsi.c b/sys/dev/pv/vioscsi.c > index 3ebac61abee..9375a33eef0 100644 > --- a/sys/dev/pv/vioscsi.c > +++ b/sys/dev/pv/vioscsi.c > @@ -433,7 +433,7 @@ vioscsi_alloc_reqs(struct vioscsi_softc *sc, struct virtio_softc *vsc, > > allocsize = nreqs * sizeof(struct vioscsi_req); > r = bus_dmamem_alloc(vsc->sc_dmat, allocsize, 0, 0, > - &sc->sc_reqs_segs[0], 1, &rsegs, BUS_DMA_NOWAIT); > + &sc->sc_reqs_segs[0], 1, &rsegs, BUS_DMA_NOWAIT | BUS_DMA_64BIT); > if (r != 0) { > printf("bus_dmamem_alloc, size %zd, error %d\n", > allocsize, r); > @@ -486,13 +486,16 @@ vioscsi_alloc_reqs(struct vioscsi_softc *sc, struct virtio_softc *vsc, > r = bus_dmamap_create(vsc->sc_dmat, > offsetof(struct vioscsi_req, vr_xs), 1, > offsetof(struct vioscsi_req, vr_xs), 0, > - BUS_DMA_NOWAIT|BUS_DMA_ALLOCNOW, &vr->vr_control); > + BUS_DMA_NOWAIT | BUS_DMA_ALLOCNOW | BUS_DMA_64BIT, > + &vr->vr_control); > if (r != 0) { > printf("bus_dmamap_create vr_control failed, error %d\n", r); > return i; > } > r = bus_dmamap_create(vsc->sc_dmat, MAXPHYS, SEG_MAX, > - MAXPHYS, 0, BUS_DMA_NOWAIT|BUS_DMA_ALLOCNOW, &vr->vr_data); > + MAXPHYS, 0, > + BUS_DMA_NOWAIT | BUS_DMA_ALLOCNOW | BUS_DMA_64BIT, > + &vr->vr_data); nit: I think you can avoid an extra line by putting MAXPHYS on the first line? > if (r != 0) { > printf("bus_dmamap_create vr_data failed, error %d\n", r ); > return i; > diff --git a/sys/dev/pv/virtio.c b/sys/dev/pv/virtio.c > index aa63fd5e9ba..759bf05ccb8 100644 > --- a/sys/dev/pv/virtio.c > +++ b/sys/dev/pv/virtio.c > @@ -387,9 +387,15 @@ virtio_alloc_vq(struct virtio_softc *sc, struct virtqueue *vq, int index, > allocsize3 = 0; > allocsize = allocsize1 + allocsize2 + allocsize3; > > - /* alloc and map the memory */ > - r = bus_dmamem_alloc(sc->sc_dmat, allocsize, VIRTIO_PAGE_SIZE, 0, > - &vq->vq_segs[0], 1, &rsegs, BUS_DMA_NOWAIT); > + /* > + * alloc and map the memory > + * > + * With virtio 0.9, the ring memory must be in the lowest 2^32 > + * pages. For simplicity, we use this limit even for virtio 1.0. > + */ > + r = bus_dmamem_alloc_range(sc->sc_dmat, allocsize, VIRTIO_PAGE_SIZE, 0, > + &vq->vq_segs[0], 1, &rsegs, BUS_DMA_NOWAIT, 0, > + 0xfffffffffff); So in principe the upper limit would depend on VIRTIO_PAGE_SIZE. So should this be something like ((uint64_t)VIRTIO_PAGE_SIZE << 32) - 1) instead. And does this work on 32-bit architectures? > if (r != 0) { > printf("virtqueue %d for %s allocation failed, error %d\n", > index, name, r); > @@ -403,7 +409,7 @@ virtio_alloc_vq(struct virtio_softc *sc, struct virtqueue *vq, int index, > goto err; > } > r = bus_dmamap_create(sc->sc_dmat, allocsize, 1, allocsize, 0, > - BUS_DMA_NOWAIT, &vq->vq_dmamap); > + BUS_DMA_NOWAIT | BUS_DMA_64BIT, &vq->vq_dmamap); > if (r != 0) { > printf("virtqueue %d for %s dmamap creation failed, " > "error %d\n", index, name, r); > >