[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] one question on the PCI ioport base address forhvm domain in hvmloader.c
I don't think direct mapping is important. Today every i/o port access in a PV driver domain traps to the hypervisor, and we don't see a significant overhead for doing this with any non-ancient hardware, because i/o ports are not used on data paths much these days. Of course a VMEXIT is rather more expensive than a #GP, but still I think this is an optimisation that can wait. At the moment most people are unable to make Xen + VT-d work with their devices at all, let alone with 99.999% of fully native performance. ;-) -- Keir On 7/1/08 15:52, "Jiang, Yunhong" <yunhong.jiang@xxxxxxxxx> wrote: > Yes, the guest OS should be allowed to remap BAR. > But I don't know are there any OS will try to remap BAR, except Vista. > And even in Vista, we can still try to disable BAR remapping through > bcdedit. > The benifit to set guest IO port address same as physical one is, if > guest didn't try to remap, then it can access IO port directly without > cause VMExit. That may help performance. Although IO port is not so > important for PCI device now, but that may still be helpful on some > situation, considering USB 1.1 device. > Of course,we need still support the BAR remapping, what we can do is, > our guest BIOS (hvmloader in fact) will set the initial value same to > physical one. > > > -- Yunhong Jiang > > xen-devel-bounces@xxxxxxxxxxxxxxxxxxx <> wrote: >> Why not keep the level of indirection? After all, the guest OS should > be >> allowed to remap any BAR and we should support that. >> >> -- Keir >> >> On 7/1/08 11:47, "Jiang, Yunhong" <yunhong.jiang@xxxxxxxxx> wrote: >> >>> I'm considering to make guest IO port address same as physical one > for >>> passthrough hvm domain. However, in hvmloader.c's pci_setup(), the >>> io_base is hardcoded as 0xc000, are there any special reason for this >>> value? (I checked and seems it comes from original qemu's code) >>> >>> Can anyone give me some hints on it? >>> >>> Thanks >>> -- Yunhong Jiang >>> >>> _______________________________________________ >>> Xen-devel mailing list >>> Xen-devel@xxxxxxxxxxxxxxxxxxx >>> http://lists.xensource.com/xen-devel >> >> >> >> _______________________________________________ >> Xen-devel mailing list >> Xen-devel@xxxxxxxxxxxxxxxxxxx >> http://lists.xensource.com/xen-devel _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxxxxxxxx http://lists.xensource.com/xen-devel
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