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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH 0/2 for-4.12] Introduce runstate area registration with phys address
On 08/03/2019 11:15, Julien Grall wrote: > Hi Juergen, > > On 3/8/19 6:28 AM, Juergen Gross wrote: >> On 07/03/2019 19:00, Julien Grall wrote: >>> Hi Roger, >>> >>> On 07/03/2019 17:15, Roger Pau Monné wrote: >>>> On Thu, Mar 07, 2019 at 04:36:59PM +0000, Julien Grall wrote: >>>>> Hi Roger, >>>>> >>>>> Thank you for the answer. >>>>> >>>>> On 07/03/2019 16:16, Roger Pau Monné wrote: >>>>>> On Thu, Mar 07, 2019 at 03:17:54PM +0000, Julien Grall wrote: >>>>>>> Hi Andrii, >>>>>>> >>>>>>> On 07/03/2019 14:34, Andrii Anisov wrote: >>>>>>>> On 07.03.19 16:02, Julien Grall wrote: >>>>>>>>>> - IMHO, this implementation is simpler and cleaner than >>>>>>>>>> what I >>>>>>>>>> have for runstate mapping on access. >>>>>>>>> >>>>>>>>> Did you implement it using access_guest_memory_by_ipa? >>>>>>>> Not exactly, access_guest_memory_by_ipa() has no implementation >>>>>>>> for x86. >>>>>>>> But it is made around that code. >>>>>>> >>>>>>> For the HVM, the equivalent function is hvm_copy_to_guest_phys. I >>>>>>> don't know >>>>>>> what would be the interface for PV. Roger, any idea? >>>>>> >>>>>> For PV I think you will have to use get_page_from_gfn, check the >>>>>> permissions, map it, write and unmap it. The same flow would also >>>>>> work >>>>>> for HVM, so I'm not sure if there's much point in using >>>>>> hvm_copy_to_guest_phys. Or you can implement a generic >>>>>> copy_to_guest_phys helper that works for both PV and HVM. >>>>>> >>>>>> Note that for translated guests you will have to walk the HAP page >>>>>> tables for each vCPU for each context switch, which I think will be >>>>>> expensive in terms of performance (I might be wrong however, since I >>>>>> have no proof of this). >>>>> >>>>> AFAICT, we already walk the page-table with the current >>>>> implementation. So >>>>> this should be no different here, except we will not need to walk the >>>>> guest-PT here. No? >>>> >>>> Yes, current implementation is even worse because it walks both the >>>> guest page tables and the HAP page tables in the HVM case. It would be >>>> interesting IMO if we could avoid walking any of those page tables. >>>> >>>> I see you have concerns about permanently mapping the runstate area, >>>> so I'm not going to oppose, albeit even with only 1G of VA space you >>>> can map plenty of runstate areas, and taking into account this is >>>> 32bit hardware I'm not sure you will ever have that many vCPUs that >>>> you will run out of VA space to map runstate areas. >>> >>> Actually the vmap is only 768MB. The vmap is at the moment used for >>> mapping: >>> - MMIO devices (through ioremap) >>> - event channel pages >>> >>> As the runstate is far smaller than a page, this sounds like a waste of >>> memory for a benefits that haven't not yet been shown. Indeed, number >>> provided by Andrii either show worst performance or similar one. >>> >>> But TBH, I am not expecting that a really clear performance improvement >>> on Arm as there are a lot to do in the context switch. >>> >>>> >>>> That being said, if the implementation turns out to be more >>>> complicated because of this permanent mapping, walking the guest HAP >>>> page tables is certainly no worse than what's done ATM. >>> >>> To be honest I am not fully against always mapping the runstate in Xen. >>> But I need data to show this is worth it. So far, the performance >>> promised are not there and the implementation is not foolproof yet. >>> >>> If we want to keep the runstate mapped permanently, then we need to add >>> either a lock or a refcounting. So the page does not disappear during >>> context switch if we happen to update the runstate concurrently (via the >>> hypercall). >>> >>> This may increase the complexity of the implementation (not sure by how >>> much thought). >>> >>> Another solution is to prevent the runstate to be updated. But I think >>> we will just add a bit more burden in the guest OS. >> >> Not sure about other systems, but current Linux kernel registers the >> runstate area for other cpus only if those are not up. So there is no >> way the runstate of that foreign vcpu could be updated during >> registering it. Of course this would need to be tested (e.g. -EBUSY >> for registering runstate of an active foreign vcpu). > > I was under the impression that xen_vcpu_restore() (see > arch/x86/xen/enlighten.c) may update the runstate while the vCPU is up > (at least on pre-Xen 4.5). Did I miss anything? Pre Xen 4.5: yes. But I don't think you want to introduce the new semantics in Xen 4.4 and older? Juergen _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxxxxxxxxx https://lists.xenproject.org/mailman/listinfo/xen-devel
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