[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] One question about the hypercall to translate gfn to mfn.
> -----Original Message----- > From: Ian Campbell > Sent: 09 December 2014 11:29 > To: Paul Durrant > Cc: Tim (Xen.org); Yu, Zhang; Kevin Tian; Keir (Xen.org); JBeulich@xxxxxxxx; > Xen-devel@xxxxxxxxxxxxx > Subject: Re: [Xen-devel] One question about the hypercall to translate gfn to > mfn. > > On Tue, 2014-12-09 at 11:17 +0000, Paul Durrant wrote: > > > -----Original Message----- > > > From: Ian Campbell > > > Sent: 09 December 2014 11:11 > > > To: Paul Durrant > > > Cc: Tim (Xen.org); Yu, Zhang; Kevin Tian; Keir (Xen.org); > JBeulich@xxxxxxxx; > > > Xen-devel@xxxxxxxxxxxxx > > > Subject: Re: [Xen-devel] One question about the hypercall to translate > gfn to > > > mfn. > > > > > > On Tue, 2014-12-09 at 11:05 +0000, Paul Durrant wrote: > > > > > -----Original Message----- > > > > > From: Tim Deegan [mailto:tim@xxxxxxx] > > > > > Sent: 09 December 2014 10:47 > > > > > To: Yu, Zhang > > > > > Cc: Paul Durrant; Keir (Xen.org); JBeulich@xxxxxxxx; Kevin Tian; Xen- > > > > > devel@xxxxxxxxxxxxx > > > > > Subject: Re: One question about the hypercall to translate gfn to mfn. > > > > > > > > > > At 18:10 +0800 on 09 Dec (1418145055), Yu, Zhang wrote: > > > > > > Hi all, > > > > > > > > > > > > As you can see, we are pushing our XenGT patches to the > upstream. > > > One > > > > > > feature we need in xen is to translate guests' gfn to mfn in XenGT > dom0 > > > > > > device model. > > > > > > > > > > > > Here we may have 2 similar solutions: > > > > > > 1> Paul told me(and thank you, Paul :)) that there used to be a > > > > > > hypercall, XENMEM_translate_gpfn_list, which was removed by > Keir in > > > > > > commit 2d2f7977a052e655db6748be5dabf5a58f5c5e32, because > there > > > was > > > > > no > > > > > > usage at that time. > > > > > > > > > > It's been suggested before that we should revive this hypercall, and I > > > > > don't think it's a good idea. Whenever a domain needs to know the > > > > > actual MFN of another domain's memory it's usually because the > > > > > security model is problematic. In particular, finding the MFN is > > > > > usually followed by a brute-force mapping from a dom0 process, or by > > > > > passing the MFN to a device for unprotected DMA. > > > > > > > > > > These days DMA access should be protected by IOMMUs, or else > > > > > the device drivers (and associated tools) are effectively inside the > > > > > hypervisor's TCB. Luckily on x86 IOMMUs are widely available (and > > > > > presumably present on anything new enough to run XenGT?). > > > > > > > > > > So I think the interface we need here is a please-map-this-gfn one, > > > > > like the existing grant-table ops (which already do what you need by > > > > > returning an address suitable for DMA). If adding a grant entry for > > > > > every frame of the framebuffer within the guest is too much, maybe > we > > > > > can make a new interface for the guest to grant access to larger > > > > > areas. > > > > > > > > > > > > > IIUC the in-guest driver is Xen-unaware so any grant entry would have > > > > to be put in the guests table by the tools, which would entail some > > > > form of flexibly sized reserved range of grant entries otherwise any > > > > PV driver that are present in the guest would merrily clobber the new > > > > grant entries. > > > > A domain can already priv map a gfn into the MMU, so I think we just > > > > need an equivalent for the IOMMU. > > > > > > I'm not sure I'm fully understanding what's going on here, but is a > > > variant of XENMEM_add_to_physmap+XENMAPSPACE_gmfn_foreign > which > > > also > > > returns a DMA handle a plausible solution? > > > > > > > I think we want be able to avoid setting up a PTE in the MMU since > > it's not needed in most (or perhaps all?) cases. > > Another (wildly under-informed) thought then: > > A while back Global logic proposed (for ARM) an infrastructure for > allowing dom0 drivers to maintain a set of iommu like pagetables under > hypervisor supervision (they called these "remoteprocessor iommu"). > > I didn't fully grok what it was at the time, let alone remember the > details properly now, but AIUI it was essentially a framework for > allowing a simple Xen side driver to provide PV-MMU-like update > operations for a set of PTs which were not the main-processor's PTs, > with validation etc. > > See http://thread.gmane.org/gmane.comp.emulators.xen.devel/212945 > > The introductory email even mentions GPUs... > That series does indeed seem to be very relevant. Paul > Ian. _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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