On 2016/10/4 17:05, Stefan Hajnoczi wrote: > On Tue, Oct 04, 2016 at 02:53:25AM +0000, gong lei wrote: >> Hi Stefan, >> >> Thanks for your comments. >> >>> On Wed, Sep 28, 2016 at 05:08:24PM +0800, Gonglei wrote: >>>> /+For scatter/gather list support, a buffer can be represented by / >>>> /virtio_crypto_iovec structure./ >>>> /+/ >>>> /+The structure is defined as follows:/ >>>> /+/ >>>> /+\begin{lstlisting}/ >>>> /+struct virtio_crypto_iovec {/ >>>> /+ /* Guest physical address *// >>>> /+ le64 addr;/ >>>> /+ /* Length of guest physical address *// >>>> /+ le32 len;/ >>>> /+/* This marks a buffer as continuing via the next field *// >>>> /+#define VIRTIO_CRYPTO_IOVEC_F_NEXT 1/ >>>> /+ /* The flags as indicated above VIRTIO_CRYPTO_IOVEC_F_*. *// >>>> /+ le32 flags;/ >>>> /+ /* Pointer to next struct virtio_crypto_iovec if flags & NEXT *// >>>> /+ le64 next_iovec;/ >>>> /+};/ >>>> /+\end{lstlisting}/ >>> The vring already provides scatter-gather I/O. It is usually not >>> necessary to define scatter-gather I/O at the device level. Addresses >>> can be translated by the virtio transport (PCI, MMIO, CCW). For example >>> PCI bus addresses with IO-MMU. Therefore it's messy to use guest >>> physical addresses in the device specification. >>> >>>> /+Each data request uses virtio_crypto_hash_data_req structure to >>>> store / >>>> /information/ >>>> /+used to run the HASH operations. The request only occupies one entry/ >>>> /+in the Vring Descriptor Table in the virtio crypto device's dataq, >>>> which / >>>> /improves/ >>>> /+the throughput of data transmitted for the HASH service, so that the >>> virtio / >>>> /crypto/ >>>> /+device can be better accelerated./ >>> Indirect vrings also only require one entry in the descriptor table. I >>> don't understand why you are reinventing scatter-gather I/O. >>> >>> Am I missing something? >> Yes, I knew indirect vring. But for virtio-crypto device' request, each >> crypto request include >> many buffers. Take algorithm chain' request as an examle, the driver >> need to transmit source >> data, dstination data, initializaion vector, additional authentication >> data, digest result >> data etc. to the device. In those data, the source data and destionation >> data etc. may be composed >> by scatter-gather I/O. That's the background. >> >> In virtio-crypto spec, we use a structure to store all those contents so >> that we can put all those data >> into one entry in the Descriptor Table, otherwise the effect of >> acceleration is not good. We >> thought other methods to inprove the performance as well, such as >> increasing the virng size >> of dataq, but the maxinum is 1024 at present, and it maybe influence the >> latency if the vring >> size is too big. >> >> For the indirect ving in existing Virtio spec, is only used for one >> buffer, which can't cover >> our situation. > Indirect vring uses 1 descriptor in the vring descriptor table, but that > descriptor points to a separate table that can have many descriptors. > You are not limited to just 1 scatter-gather element. > > Also, I've noticed that the structs in the spec are mixing > device-readable and device-writable fields in structs. This is not > allowed since virtio requires that all device-readable data comes before > all all device-writable data. The spec 2.4.5 says something related, but not forced.
A device MUST NOT write to a device-readable buffer, and a device SHOULD NOT read a device-writable buffer (it MAY do so for debugging or diagnostic purposes). > I think you'll be able to use indirect vring with the same performance > as customer scatter-gather I/O once you think through the layout of > device-readable and device-writable data. > > In order to lay out requests correctly the device-readable headers > struct must contain the length of all variable-sized fields. > > For example, the header would have an iv_len field and the device will > expect that number of bytes after the header struct: > > Device-readable: > [header] > [iv] > [input data] > > Device-writeable: > [output data] > [return/error values] > > One more thing to keep in mind is that according to the VIRTIO > specification the device must not rely on the framing (or boundaries of > scatter-gather elements). In the example above it means that the [iv] > could either be a single vring_desc or it could be multiple descs, or it > could share vring_descs with [header] and/or [input data]. > > In other words, the device emulation code must not assume elem->in[0] is > [header], elem->in[1] is [iv], and elem->in[2] is [input data]. It has > to process without giving special meaning to scatter-gather buffer size. > > Stefan Yes, I knew that and I did that last year, but I didn't get the good performance unfortunately. Because we need to handle multiple buffers respectively on one request, which add the overhead of software layer. It's obviously for vhost-user cryptodev backend in DPDK. Maybe I didn't express my meaning clearly. Let's assume that a have 6 buffers in one crypto request: BufA: output data, a single buffer BufB: output data, a scatter-gather I/O BufC: output data, a single buffer BufD: in data, a scatter-gather I/O BufE: in data, a single buffer BufF: in data, a single buffer We need following steps to translate the request to the device: 1. add_outbuf() for BufA, which will occupy one entry in the descryptor table 2.add_outbuf() for BufB, which will occupy one entry in the descryptor table if using indirect vring, or multiple entries if not using indirecting vring 3. add_outbuf() for BufC, which will occupy one entry in the descryptor table 4. add_inbuf() for BufD, which will occupy one entry in the descryptor table if using indirect vring, or multiple entries if not using indirecting vring 5. add_inbuf() for BufE, which will occupy one entry in the descryptor table 6. add_outbuf() for BufF, which will occupy one entry in the descryptor table It means that one request will occupy 6 entries at least. Am I right? -- Regards, -Gonglei