On Fri, Jan 27, 2023 at 02:47:55PM +0200, Alexander Shishkin wrote: > "Michael S. Tsirkin" <mst@xxxxxxxxxx> writes: > > > On Fri, Jan 27, 2023 at 01:55:43PM +0200, Alexander Shishkin wrote: > >> "Michael S. Tsirkin" <mst@xxxxxxxxxx> writes: > >> > >> > On Thu, Jan 19, 2023 at 10:13:18PM +0200, Alexander Shishkin wrote: > >> >> When handling control messages, instead of peeking at the device memory > >> >> to obtain bits of the control structure, > >> > > >> > Except the message makes it seem that we are getting data from > >> > device memory, when we do nothing of the kind. > >> > >> We can be, see below. > >> > >> >> take a snapshot of it once and > >> >> use it instead, to prevent it from changing under us. This avoids races > >> >> between port id validation and control event decoding, which can lead > >> >> to, for example, a NULL dereference in port removal of a nonexistent > >> >> port. > >> >> > >> >> The control structure is small enough (8 bytes) that it can be cached > >> >> directly on the stack. > >> > > >> > I still have no real idea why we want a copy here. > >> > If device can poke anywhere at memory then it can crash kernel anyway. > >> > If there's a bounce buffer or an iommu or some other protection > >> > in place, then this memory can no longer change by the time > >> > we look at it. > >> > >> We can have shared pages between the host and guest without bounce > >> buffers in between, so they can be both looking directly at the same > >> page. > >> > >> Regards, > > > > How does this configuration work? What else is in this page? > > So, for example in TDX, you have certain pages as "shared", as in > between guest and hypervisor. You can have virtio ring(s) in such > pages. That one's marked as dma coherent. > It's likely that there'd be a swiotlb buffer there instead, but > sharing pages between host virtio and guest virtio drivers is possible. It's not something console does though, does it? > Apologies if the language is confusing, I hope I'm answering the > question. > > Regards, > -- > Alex I'd like an answer to when does the console driver share the buffer in question, not when generally some pages shared. -- MST _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
