On Fri, Feb 23, 2018 at 06:35:30PM +0100, Paolo Bonzini wrote: > On 23/02/2018 18:22, Konrad Rzeszutek Wilk wrote: > > On Fri, Feb 23, 2018 at 10:37:49AM +0100, Paolo Bonzini wrote: > >> On 22/02/2018 18:07, Konrad Rzeszutek Wilk wrote: > >>>> Having a paravirt indirect call in the IBRS restore path is not a > >>>> good idea, since we are trying to protect from speculative execution > >>>> of bogus indirect branch targets. It is also slower, so use > >>>> native_wrmsrl on the vmentry path too. > >>> But it gets replaced during patching. As in once the machine boots > >>> the assembler changes from: > >>> > >>> callq *0xfffflbah > >>> > >>> to > >>> wrmsr > >>> > >>> ? I don't think you need this patch. > >> > >> Why not be explicit? According to the spec, PRED_CMD and SPEC_CTRL > > > > Explicit is fine. > > > > But I would recommend you change the commit message to say so, and > > perhaps remove 'It is also slower' - as that is incorrect. > > Actually it is faster---that's why I made the change in the first place, > though later I noticed > > > If it is detected to be Xen PV, then yes > > it will be a call to a function. But that won't help as Xen PV runs in > > ring 3, so it has a whole bunch of other issues. > > Ok, I wasn't sure about PVH (which runs in ring 0 afair).
Right. PVH is HVM without any emulated devices or BIOSes or such. In the context of the paravirt ops, Xen PVH == Xen HVM. Xen PV (and lguests) are the only ones that patch the the callq *0xffffblah to callq 0xffff800 While everyone else does the wrmsr. > > Paolo