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[79.176.51.222]) by smtp.gmail.com with ESMTPSA id o13sm4347586wrm.74.2020.04.06.07.34.42 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 06 Apr 2020 07:34:43 -0700 (PDT) Date: Mon, 6 Apr 2020 10:34:40 -0400 From: "Michael S. Tsirkin" To: Jason Wang Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org, virtualization@lists.linux-foundation.org, netdev@vger.kernel.org Subject: Re: [PATCH] vhost: force spec specified alignment on types Message-ID: <20200406102531-mutt-send-email-mst@kernel.org> References: <20200406124931.120768-1-mst@redhat.com> <045c84ed-151e-a850-9c72-5079bd2775e6@redhat.com> <20200406095424-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Sender: netdev-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On Mon, Apr 06, 2020 at 10:09:27PM +0800, Jason Wang wrote: > > On 2020/4/6 下午9:55, Michael S. Tsirkin wrote: > > On Mon, Apr 06, 2020 at 09:34:00PM +0800, Jason Wang wrote: > > > On 2020/4/6 下午8:50, Michael S. Tsirkin wrote: > > > > The ring element addresses are passed between components with different > > > > alignments assumptions. Thus, if guest/userspace selects a pointer and > > > > host then gets and dereferences it, we might need to decrease the > > > > compiler-selected alignment to prevent compiler on the host from > > > > assuming pointer is aligned. > > > > > > > > This actually triggers on ARM with -mabi=apcs-gnu - which is a > > > > deprecated configuration, but it seems safer to handle this > > > > generally. > > > > > > > > I verified that the produced binary is exactly identical on x86. > > > > > > > > Signed-off-by: Michael S. Tsirkin > > > > --- > > > > > > > > This is my preferred way to handle the ARM incompatibility issues > > > > (in preference to kconfig hacks). > > > > I will push this into next now. > > > > Comments? > > > > > > I'm not sure if it's too late to fix. It would still be still problematic > > > for the userspace that is using old uapi headers? > > > > > > Thanks > > It's not a problem in userspace. The problem is when > > userspace/guest uses 2 byte alignment and passes it to kernel > > assuming 8 byte alignment. The fix is for host not to > > make these assumptions. > > > Yes, but I meant when userspace is complied with apcs-gnu, then it still > assumes 8 byte alignment? > > Thanks That's not a problem since with vhost userspace is doing the allocation. So it can increase alignment with no bad effect. I agree it's probably safest not to touch struct vring at all though. > > > > > > > drivers/vhost/vhost.h | 6 ++--- > > > > include/uapi/linux/virtio_ring.h | 41 ++++++++++++++++++++++++-------- > > > > 2 files changed, 34 insertions(+), 13 deletions(-) > > > > > > > > diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h > > > > index cc82918158d2..a67bda9792ec 100644 > > > > --- a/drivers/vhost/vhost.h > > > > +++ b/drivers/vhost/vhost.h > > > > @@ -74,9 +74,9 @@ struct vhost_virtqueue { > > > > /* The actual ring of buffers. */ > > > > struct mutex mutex; > > > > unsigned int num; > > > > - struct vring_desc __user *desc; > > > > - struct vring_avail __user *avail; > > > > - struct vring_used __user *used; > > > > + vring_desc_t __user *desc; > > > > + vring_avail_t __user *avail; > > > > + vring_used_t __user *used; > > > > const struct vhost_iotlb_map *meta_iotlb[VHOST_NUM_ADDRS]; > > > > struct vhost_desc *descs; > > > > diff --git a/include/uapi/linux/virtio_ring.h b/include/uapi/linux/virtio_ring.h > > > > index 559f42e73315..cd6e0b2eaf2f 100644 > > > > --- a/include/uapi/linux/virtio_ring.h > > > > +++ b/include/uapi/linux/virtio_ring.h > > > > @@ -118,16 +118,6 @@ struct vring_used { > > > > struct vring_used_elem ring[]; > > > > }; > > > > -struct vring { > > > > - unsigned int num; > > > > - > > > > - struct vring_desc *desc; > > > > - > > > > - struct vring_avail *avail; > > > > - > > > > - struct vring_used *used; > > > > -}; > > > > - > > > > /* Alignment requirements for vring elements. > > > > * When using pre-virtio 1.0 layout, these fall out naturally. > > > > */ > > > > @@ -164,6 +154,37 @@ struct vring { > > > > #define vring_used_event(vr) ((vr)->avail->ring[(vr)->num]) > > > > #define vring_avail_event(vr) (*(__virtio16 *)&(vr)->used->ring[(vr)->num]) > > > > +/* > > > > + * The ring element addresses are passed between components with different > > > > + * alignments assumptions. Thus, we might need to decrease the compiler-selected > > > > + * alignment, and so must use a typedef to make sure the __aligned attribute > > > > + * actually takes hold: > > > > + * > > > > + * https://gcc.gnu.org/onlinedocs//gcc/Common-Type-Attributes.html#Common-Type-Attributes > > > > + * > > > > + * When used on a struct, or struct member, the aligned attribute can only > > > > + * increase the alignment; in order to decrease it, the packed attribute must > > > > + * be specified as well. When used as part of a typedef, the aligned attribute > > > > + * can both increase and decrease alignment, and specifying the packed > > > > + * attribute generates a warning. > > > > + */ > > > > +typedef struct vring_desc __attribute__((aligned(VRING_DESC_ALIGN_SIZE))) > > > > + vring_desc_t; > > > > +typedef struct vring_avail __attribute__((aligned(VRING_AVAIL_ALIGN_SIZE))) > > > > + vring_avail_t; > > > > +typedef struct vring_used __attribute__((aligned(VRING_USED_ALIGN_SIZE))) > > > > + vring_used_t; > > > > + > > > > +struct vring { > > > > + unsigned int num; > > > > + > > > > + vring_desc_t *desc; > > > > + > > > > + vring_avail_t *avail; > > > > + > > > > + vring_used_t *used; > > > > +}; > > > > + > > > > static inline void vring_init(struct vring *vr, unsigned int num, void *p, > > > > unsigned long align) > > > > {