Re: [PATCH 2/4] KVM: selftests: Increase UCALL_MAX_ARGS to 7

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On Sat, Jun 18, 2022 at 12:09:11AM +0000, Sean Christopherson wrote:
> On Fri, Jun 17, 2022, Colton Lewis wrote:
> > On Thu, Jun 16, 2022 at 02:10:06PM +0200, Andrew Jones wrote:
> > > We probably want to ensure all architectures are good with this. afaict,
> > > riscv only expects 6 args and uses UCALL_MAX_ARGS to cap the ucall inputs,
> > > for example.
> > 
> > All architectures use UCALL_MAX_ARGS for that. Are you saying there
> > might be limitations beyond the value of the macro? If so, who should
> > verify whether this is ok?
> 
> I thought there were architectural limitations too, but I believe I was thinking
> of vcpu_args_set(), where the number of params is limited by the function call
> ABI, e.g. the number of registers.
> 
> Unless there's something really, really subtle going on, all architectures pass
> the actual ucall struct purely through memory.  Actually, that code is ripe for
> deduplication, and amazingly it doesn't conflict with Colton's series.  Patches
> incoming...
>

RISC-V uses sbi_ecall() for their implementation of ucall(). CC'ing Anup
for confirmation, but if I understand the SBI spec correctly, then inputs
are limited to registers a0-a5.

Thanks,
drew




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