Re: çå: [PATCH] arm64/ptrace: add PTRACE_SYSEMU and PTRACE_SYSEMU_SINGLESTEP support

[Richard Weinberger]

Am Dienstag, 4. September 2018, 04:11:07 CEST schrieb Haibo Xu (Arm Technology China):
> Hi Richard,
> 
> What do you mean by done it in the core? moving macro definition to include/uapi/linux/ptrace.h?
> The patch is strictly follow x86's sematic on PTRACE_SYSEMU/SINGLESTEP support.

Well, the feature itself is not really architecture specific.
Just because x86 does it in arch/x86, it does not mean that this is the best way.
I guess this is also what Will tried to say.
If we can, we should implement PTRACE_SYSEMU in the core ptrace code and not
per architecture.

> > > I wonder what Haibo Xu want to do with PTRACE_SYSEMU on arm64.
> > > Are you porting UML or gvisor to arm64?
> >
> > That's a good question. Haibo?
> 
> The story is we are working on a container runtime(Google Gvisor) support on ARM64 platform,
> and the Gvisor depend on Linux kernel PTRACE_SYSEMU/SINGLESTEP support.

Gvisor also supports a kvm backend which should be *much* faster than PTRACE_SYSEMU.
Otherwise gvisor suffers from the same performance drawbacks as UML does.
Pagefaults via SIGSEGV/mmap, syscall gate via ptrace().

Did you check, is PTRACE_SYSEMU really the way to go for gvisor?
Last time I checked the KVM backend looked promising but still WIP, though.

I also wonder whether PTRACE_SYSEMU is really the only missing bit to support
gvisor on arm64. Did you check how to work around VIPT/VIVT caching issues?
UML (and gvisor in this context) have lots of implicit x86 dependencies.

Thanks,
//richard