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Re: [Xen-devel] [PATCH] Re-enable LTR/OBFF when device is owned by pciback

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> > Qemu only emulate the device which is assigned to guest, but it does not
> > emulate the physic device's whole bus path. Seems allowing guest request
> > enabling looks reasonable, but I think it's only applicable for "per device
> > feature(that unnecessary to enable the whole pci path)".
> No - if the guest asks the host to call pci_enable_ltr(), it'll imply enabling
> for the entire path (as that's what the call results in). Which imo is better
> than enabling a feature upfront, just in case.

For purpose of guest asking the host to call pci_enable_ltr(), there are two 
probable ways:
a. According to PCIE spec, "software must not enable LTR in an Endpoint unless 
the Root Complex and all intermediate Switches indicate support for LTR". That 
means Qemu has to emulate the entire path of platform, so that guest can detect 
whether endpoint device can be enabled firstly.
b. using pv guest mode, let the host call pci_enable_ltr().

Obviously, the two ways are not very good for this case. For the first way, we 
can emulate the entire path when qemu support PCIE emulation. But currently 
it's better to enable ltr/obff in pciback driver.


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