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Tsirkin" To: Eugenio Perez Martin Cc: Jason Wang , leiyang@redhat.com, Laurent Vivier , sgarzare@redhat.com, Zhu Lingshan , virtualization@lists.linux-foundation.org, si-wei.liu@oracle.com, linux-kernel@vger.kernel.org, lulu@redhat.com, Gautam Dawar , alvaro.karsz@solid-run.com, "Longpeng (Mike, Cloud Infrastructure Service Product Dept.)" Subject: Re: [PATCH 2/2] vringh: fetch used_idx from vring at vringh_init_iotlb Message-ID: <20230213070248-mutt-send-email-mst@kernel.org> References: <20230118164359.1523760-1-eperezma@redhat.com> <20230118164359.1523760-3-eperezma@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jan 31, 2023 at 08:58:37AM +0100, Eugenio Perez Martin wrote: > On Tue, Jan 31, 2023 at 4:16 AM Jason Wang wrote: > > > > On Tue, Jan 31, 2023 at 12:39 AM Eugenio Perez Martin > > wrote: > > > > > > On Sun, Jan 29, 2023 at 7:01 AM Jason Wang wrote: > > > > > > > > On Thu, Jan 19, 2023 at 4:11 PM Eugenio Perez Martin > > > > wrote: > > > > > > > > > > On Thu, Jan 19, 2023 at 4:20 AM Jason Wang wrote: > > > > > > > > > > > > On Thu, Jan 19, 2023 at 12:44 AM Eugenio Pérez wrote: > > > > > > > > > > > > > > Starting from an used_idx different than 0 is needed in use cases like > > > > > > > virtual machine migration. Not doing so and letting the caller set an > > > > > > > avail idx different than 0 causes destination device to try to use old > > > > > > > buffers that source driver already recover and are not available > > > > > > > anymore. > > > > > > > > > > > > > > While callers like vdpa_sim set avail_idx directly it does not set > > > > > > > used_idx. Instead of let the caller do the assignment, fetch it from > > > > > > > the guest at initialization like vhost-kernel do. > > > > > > > > > > > > > > To perform the same at vring_kernel_init and vring_user_init is left for > > > > > > > the future. > > > > > > > > > > > > > > Signed-off-by: Eugenio Pérez > > > > > > > --- > > > > > > > drivers/vhost/vringh.c | 25 +++++++++++++++++++++++-- > > > > > > > 1 file changed, 23 insertions(+), 2 deletions(-) > > > > > > > > > > > > > > diff --git a/drivers/vhost/vringh.c b/drivers/vhost/vringh.c > > > > > > > index 33eb941fcf15..0eed825197f2 100644 > > > > > > > --- a/drivers/vhost/vringh.c > > > > > > > +++ b/drivers/vhost/vringh.c > > > > > > > @@ -1301,6 +1301,17 @@ static inline int putused_iotlb(const struct vringh *vrh, > > > > > > > return 0; > > > > > > > } > > > > > > > > > > > > > > +/** > > > > > > > + * vringh_update_used_idx - fetch used idx from driver's used split vring > > > > > > > + * @vrh: The vring. > > > > > > > + * > > > > > > > + * Returns -errno or 0. > > > > > > > + */ > > > > > > > +static inline int vringh_update_used_idx(struct vringh *vrh) > > > > > > > +{ > > > > > > > + return getu16_iotlb(vrh, &vrh->last_used_idx, &vrh->vring.used->idx); > > > > > > > +} > > > > > > > + > > > > > > > /** > > > > > > > * vringh_init_iotlb - initialize a vringh for a ring with IOTLB. > > > > > > > * @vrh: the vringh to initialize. > > > > > > > @@ -1319,8 +1330,18 @@ int vringh_init_iotlb(struct vringh *vrh, u64 features, > > > > > > > struct vring_avail *avail, > > > > > > > struct vring_used *used) > > > > > > > { > > > > > > > > > > > > While at this, I wonder if it's better to have a dedicated parameter > > > > > > for last_avail_idx? > > > > > > > > > > > > > > > > I also had that thought. To directly assign last_avail_idx is not a > > > > > specially elegant API IMO. > > > > > > > > > > Maybe expose a way to fetch used_idx from device vring and pass > > > > > used_idx as parameter too? > > > > > > > > If I was not wrong, we can start from last_avail_idx, for used_idx it > > > > is only needed for inflight descriptors which might require other > > > > APIs? > > > > > > > > (All the current vDPA user of vringh is doing in order processing) > > > > > > > > > > That was actually my first attempt and it works equally well for the > > > moment, but it diverges from vhost-kernel behavior for little benefit. > > > > > > To assign both values at set_vring_base mean that if vDPA introduces > > > an (hypothetical) VHOST_VDPA_F_INFLIGHT backend feature in the future, > > > the initialization process would vary a lot: > > > * Without that feature, the used_idx starts with 0, and the avail one > > > is 0 or whatever value the user set with vring_set_base. > > > * With that feature, the device will read guest's used_idx as > > > vhost-kernel? We would enable a new ioctl to set it, or expand > > > set_base to include used_idx, effectively diverting from vhost-kernel? > > > > Adding Longpeng who is looking at this. > > > > Sorry, I'll CC longpeng2@huawei.com in future series like this one. > > > We can leave this to the caller to decide. > > > > Btw, a question, at which case the device used index does not equal to > > the used index in the vring when the device is suspended? I think the > > correct way is to flush the pending used indexes before suspending. > > Otherwise we need an API to get pending used indices? > > > > > > > > To me the wisest option is to move this with vhost-kernel. Maybe we > > > need to add a feature bit to know that the hypervisor can trust the > > > device will do "the right thing" (VHOST_VDPA_F_FETCH_USED_AT_ENABLE?), > > > but we should keep it orthogonal to inflight descriptor migration in > > > my opinion. > > > > I think we need to understand if there are any other possible use > > cases for setting used idx other than inflight stuff. > > > > Answering this and the previous comment, I cannot think in any case > outside of inflight migration. I'm just trying to avoid different > behavior between vhost-kernel and vhost-vdpa, and to make features as > orthogonal as possible. > > > > > > > Having said that, I'm totally ok to do it otherwise (or to expand the > > > patch message if needed). > > > > I tend to do that in another series (not mix with the fixes). > > > > > > > > > > > > > > > > > - return vringh_init_kern(vrh, features, num, weak_barriers, > > > > > > > - desc, avail, used); > > > > > > > + int r = vringh_init_kern(vrh, features, num, weak_barriers, desc, > > > > > > > + avail, used); > > > > > > > + > > > > > > > + if (r != 0) > > > > > > > + return r; > > > > > > > + > > > > > > > + /* Consider the ring not initialized */ > > > > > > > + if ((void *)desc == used) > > > > > > > + return 0; > > > > > > > > > > > > I don't understand when we can get this (actually it should be a bug > > > > > > of the caller). > > > > > > > > > > > > > > > > You can see it in vdpasim_vq_reset. > > > > > > > > > > Note that to consider desc == 0 to be an uninitialized ring is a bug > > > > > IMO. QEMU considers it that way also, but the standard does not forbid > > > > > any ring to be at address 0. Especially if we use vIOMMU. > > > > > > > > > > So I think the best way to know if we can use the vringh is either > > > > > this way, or provide an explicit "initialized" boolean attribute. > > > > > Maybe a new "bool is_initialized(vrh)" is enough, if we don't want to > > > > > add new attributes. > > > > > > > > I wonder if we can avoid this in the simulator level instead of the > > > > vringh (anyhow it only exposes a vringh_init_xxx() helper now). > > > > > > > > > > In my opinion that is a mistake if other drivers will use it to > > > implement the emulated control virtqueue. And it requires more > > > changes. But it is doable for sure. > > > > The problem is, there's no reset API in vringh, that's why you need to > > do if ((void *)desc == used) which depends on behaviour of the vringh > > user. > > > > That's a very good point indeed. > > > So I think we should either: > > > > 1) move that check in vdpa_sim (since it's not guaranteed that all the > > vringh users will make desc equal to used during reset) > > > > or > > > > 2) introduce a vringh_reset_xxx() > > > > 1) seems a good step for -stable. > > > > We can go to 1 for sure. So let's set last_used_idx at > vdpasim_set_vq_state, same value as last_avail_idx, and stash it at > vdpasim_queue_ready. > > Do I need to resend the previous patch in this series? > > Do we need to offer a new feature flag indicating we will set used_idx > with avail_idx? > > Thanks! Jason did you forget to answer or did I miss it?