[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [XEN PATCH v2] hvmloader: Enable MMIO and I/O decode, after all resource allocation
On 14.04.2020 13:58, Paul Durrant wrote: >> -----Original Message----- >> From: Jan Beulich <jbeulich@xxxxxxxx> >> Sent: 14 April 2020 12:40 >> To: paul@xxxxxxx >> Cc: 'Shamsundara Havanur, Harsha' <havanur@xxxxxxxxxx>; >> xen-devel@xxxxxxxxxxxxxxxxxxxx; >> andrew.cooper3@xxxxxxxxxx; ian.jackson@xxxxxxxxxxxxx; wl@xxxxxxx; >> roger.pau@xxxxxxxxxx >> Subject: Re: [XEN PATCH v2] hvmloader: Enable MMIO and I/O decode, after all >> resource allocation >> >> On 14.04.2020 13:01, Paul Durrant wrote: >>>> -----Original Message----- >>>>> >>>>> Previous commit enabled MASTER for all functions. I am bit confused >>>>> here on the consensus on enabling/disabling/retaining BME. >>>>> Should we even care about MASTER? >>>> >>>> With the commit introducing its universal setting, I'm afraid to >>>> avoid regressions we can't sensibly alter the behavior unless it >>>> can be explained clearly why the original change must have been >>>> outright wrong. >>>> >>> >>> Well the original code IIRC had no justification for setting BME >>> and doing it unconditionally does seem dangerous. >> >> I'm not viewing this as dangerous, merely as (typically) pointless. >> A well behaved device won't start issuing DMA requests merely >> because it had its bus mastering capability enabled. (And in the >> context of some IOMMU work of yours you actually stated there are >> devices where clearing of this bit won't stop them from doing so.) >> > > It's a line of defence against some devices at least, What defence? Once we're past hvmloader, the guest can do whatever it wants anyway. Jan
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