On Fri, Jun 15, 2018 at 12:52:05AM +0000, Gonglei (Arei) wrote: > > > > -----Original Message----- > > From: Daniel P. Berrangé [mailto:berra...@redhat.com] > > Sent: Thursday, June 14, 2018 11:11 PM > > To: Farhan Ali <al...@linux.ibm.com> > > Cc: Halil Pasic <pa...@linux.ibm.com>; qemu-devel@nongnu.org; > > fran...@linux.ibm.com; m...@redhat.com; borntrae...@de.ibm.com; Gonglei > > (Arei) <arei.gong...@huawei.com>; longpeng <longpe...@huawei.com>; > > Viktor Mihajlovski <mihaj...@linux.vnet.ibm.com>; > > mjros...@linux.vnet.ibm.com > > Subject: Re: [Qemu-devel] [RFC v1 1/1] virtio-crypto: Allow disabling of > > cipher > > algorithms for virtio-crypto device > > > > On Thu, Jun 14, 2018 at 10:50:40AM -0400, Farhan Ali wrote: > > > > > > > > > On 06/14/2018 04:21 AM, Daniel P. Berrangé wrote: > > > > On Wed, Jun 13, 2018 at 07:28:08PM +0200, Halil Pasic wrote: > > > > > > > > > > > > > > > On 06/13/2018 05:05 PM, Daniel P. Berrangé wrote: > > > > > > On Wed, Jun 13, 2018 at 11:01:05AM -0400, Farhan Ali wrote: > > > > > > > Hi Daniel > > > > > > > > > > > > > > On 06/13/2018 05:37 AM, Daniel P. Berrangé wrote: > > > > > > > > On Tue, Jun 12, 2018 at 03:48:34PM -0400, Farhan Ali wrote: > > > > > > > > > The virtio-crypto driver currently propagates to the guest > > > > > > > > > all the cipher algorithms that the backend cryptodev can > > > > > > > > > support. But in certain cases where the guest has more > > > > > > > > > performant mechanism to handle some algorithms, it would be > > > > > > > > > useful to propagate only a subset of the algorithms. > > > > > > > > > > > > > > > > I'm not really convinced by this. > > > > > > > > > > > > > > > > The performance of crypto algorithms has many influencing > > > > > > > > factors, making it pretty hard to decide which is best > > > > > > > > without actively testing specific impls and comparing > > > > > > > > them in a manner which matches the application usage > > > > > > > > pattern. eg in theory the kernel crypto impl of an alg > > > > > > > > is faster than a userspace impl, if the kernel uses > > > > > > > > hardware accel and userspace does not. This, however, > > > > > > > > ignores the overhead of the kernel/userspace switch. > > > > > > > > The real world performance winner, thus depends on the > > > > > > > > amount of data being processed in each operation. Some > > > > > > > > times userspace can win & sometimes kernel space can > > > > > > > > win. This is even more relevant to virtio-crypto as > > > > > > > > it has more expensive context switches. > > > > > > > > > > > > > > True. But what if the guest can perform some crypto algorithms > > without a > > > > > > > incurring a VM exit? For example in s390 we have the cpacf > > instructions to > > > > > > > perform crypto and this instruction is implemented for us by our > > hardware > > > > > > > virtualization technology. In such a case it would be better not > > > > > > > to use > > > > > > > virtio-crypto's implementation of such a crypto algorithm. > > > > > > > > > > > > > > At the same time we would like to take advantage of > > > > > > > virtio-crypto's > > > > > > > acceleration capabilities for certain crypto algorithms for which > > > > > > > there > > is > > > > > > > no hardware assistance. > > > > > > > > > > > > IIUC, the kernel's crypto layer can support multiple > > > > > > implementations of > > > > > > any algorithm. Providers can report a priority against > > > > > > implementations > > > > > > which influences which impl is used in practice. So if there's a > > > > > > native > > > > > > instruction for a partiuclar algorithm I would expect the impl > > > > > > registered > > > > > > for that to be designated higher priority than other impls, so that > > > > > > it is > > > > > > used in preference to other impls. > > > > > > > > > > > > > > > > AFAIR the problem here is that in (the guest) kernel the virtio-crypto > > > > > driver has to register it's crypto algo implementations with a > > > > > priority > > > > > (single number), which dictates if it's going to be the preferred > > > > > (used) > > > > > implementation of the algorithm or not. The virtio-crypto driver does > > > > > this > > > > > without having information about the (comparative or absolute) > > performance > > > > > of it's implementation (which depends on the backend among others). I > > don't think > > > > > any dynamic re-prioritization of the algorithms takes place (e.g. > > > > > based on > > how these > > > > > perform in for the given configuration). > > > > > > > > > > I think the strategy of the virtio-crypto is to rather overstate, than > > > > > understate the performance of it's implementation. If we were to 'be > > > > > conservative' and say, 'hey we don't know nothing about the > > > > > performance, > > > > > let's make it lowest priority implementation' the implementations > > provided > > > > > by virtio-crypto would end up being used only if there is no other > > > > > implementation. And that does not sound like a good idea either. > > > > > > > > > > > > This problem you describe, however, is something that applies to *any* > > > > kerenl code that is registering a crypto algo impl for accelerator > > > > hardware. A non-virtualized crypto cards in bare metal likewise cannot > > > > assume that its AES impl is better then the host CPU's aes-ni > > > > instruction. > > > > > > > > > So the idea is to give the user the power to effectively not provide > > > > > an algorithm via virtio-crypto. That is, if the user observes a > > > > > performance > > > > > degradation because of virtio-crypto, he can turn off the bad > > > > > algorithms > > > > > at the device. That way overstatement becomes a much smaller problem. > > > > > The user can turn off the bad algorithms for reasons other than > > performance > > > > > too. > > > > > > > > > > Of course there are other ways to deal with the problem of > > > > > virtio-crypto > > > > > driver not knowing how good it's implementation of a given algo is. We > > > > > could make the in kernel crypto priorities dynamically adjustable in > > general > > > > > or we could provide the user with means to specify the priorities > > > > > (e.g. > > > > > as module parameter) with which the virtio-crypto driver registers > > > > > each > > algo. > > > > > Both of these would be knobs in the guest. It's hard to tell if these > > > > > first > > > > > one would be useful in scenarios not involving virtualization. Same > > > > > goes > > > > > for some kind of dynamic priority management for crypto algorithm > > implementations > > > > > in the Linux kernel. I assume the people involved with the respective > > > > > subsystem do not see the necessity for something like that. > > > > > > > > It still feels like this is a problem for the guest OS to solve. If you > > > > put a physical crypto accelerator in a bare metal machine, that has the > > > > same problem you describe here, so the kernel surely already needs to > > > > find > > > > a viable solution for this problem. > > > > > > > > > > How would the guest OS know which algo is better? As you mentioned it does > > > depend on few factors and the best the kernel can do is use some sort of > > > heuristics. Such a solution might not be very dynamic and might not work > > > for > > > all the cases for a user. > > > > Which is better will likely depend on the application using it. One might > > be better for use by the kernel, while another is better for use by a > > userspace application, or two userspace apps might have different > > preferences. > > > > > Shouldn't we use virtualization to give us the flexibility that we don't > > > have with physical crypto accelerator? The crypto accelerator might not > > > know > > > if it's implementation is any better, but the user can experiment and see > > > what works better. > > > > It is better to provide it all to the guest and let the guest decide which > > is best to use. If nothing else the virtio-crypto kernel module itself > > can have module parameters to control the priority it gives to each > > algorithm, or can avoid registering certain algorithms. Doing it guest > > side is more flexible, because realistically many virt host deployments > > will never give the guest admin ability to control this from the host > > side, so a guest kernel config ability will be the only thing available. > > > > From the perspective of your communication and production deployment, > I tend to agree with Daniel’s point of view. AFAICT DPDK cryptodev scheduler > PMD driver did this thing: > > " Added a packet-size based distribution mode, which distributes the enqueued > Crypto operations among two slaves, based on their data lengths." > > Which it means that the guest's driver makes the decision. > > Currently the Linux crypto framework uses the static priority of one algo to > decide > to choose what is simply. Maybe it should add a scheduler layer too?
The simplest option would probably be either a kernel module parameter, or even better, sysfs tunables, to allow the priorities to be set dynamically Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
