On Fri, 15 Nov 2019, 18:13 Andrew Cooper, <andrew.coop...@citrix.com> wrote:
> On 31/10/2019 21:25, Julien Grall wrote: > > Hi, > > > > On 31/10/2019 19:28, Andrew Cooper wrote: > >> This code is especially tangled. VCPUOP_initialise calls into > >> arch_initialise_vcpu() which calls back into default_initialise_vcpu() > which > >> is common code. > >> > >> This path is actually dead code on ARM, because VCPUOP_initialise is > filtered > >> out by do_arm_vcpu_op(). > >> > >> The only valid way to start a secondary CPU on ARM is via the PSCI > interface. > >> The same could in principle be said about INIT-SIPI-SIPI for x86 HVM, > if HVM > >> guests hadn't already interited a paravirt way of starting CPUs. > >> > >> Either way, it is quite likely that no future architectures implemented > in Xen > >> are going to want to use a PV interface, as some standardised (v)CPU > bringup > >> mechanism will already exist. > > I am not sure I agree here. Looking at Linux RISCv code (see [1] and > > [2]), it looks like the kernel has to deal with selecting one "lucky" > > CPU/hart to deal with the boot and park all the others. > > > > So it looks like to me there are nothing at the moment on RISCv to do > > (v)CPU bring-up. We might be able to use PSCI (although this is an ARM > > specific way), but would rather wait and see what RISCv folks come up > > with before deciding PV is never going to be used. > > Nothing here prohibits other architectures from using a PV interface if > they wish. > Well, your commit message and the code movement implies that nobody will ever use it. > However, your examples prove my point. There is an already-agreed way > to start RISCv CPUs which is not a PV interface, and therefore is very > unlikely to adopted to run differently under Xen. I would not call that a way to start CPUs because AFAICT all CPUs have to be brought up together and you can't offline them. This is fairly restrictive for a guest so I don't think reusing it would sustainable long term. FWIW, this is exactly what Arm used to have before PSCI. Cheers, ~Andrew >
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