Re: [PATCH v3 1/5] KVM: Add arch specific interfaces for sampling guest callchains

[Sean Christopherson <seanjc@google.com>]

On Sun, Dec 10, 2023, Marc Zyngier wrote:
> On Sun, 10 Dec 2023 08:12:18 +0000, Tianyi Liu <i.pear@outlook.com> wrote:
> > +bool kvm_arch_vcpu_read_virt(struct kvm_vcpu *vcpu, gva_t addr, void *dest, unsigned int length)
> > +{
> > +	/* TODO: implement */
> > +	return false;
> > +}
> 
> I don't do it very often, but the only thing I can say about this is
> *NAK*.
> 
> You have decided to ignore the previous review comments, which is your
> prerogative. However, I absolutely refuse to add half baked and
> *dangerous* stuff to the arm64's version of KVM.
> 
> If you can convince the x86 folks that they absolutely want this, fine
> by me. But this need to be a buy-in interface, not something that is
> required for each and every architecture to have stubs, wrongly
> suggesting that extra work is needed.
> 
> For arm64, the way to go is to have this in userspace. Which is both
> easy to implement and safe. And until we have such a userspace
> implementation as a baseline, I will not consider a kernel
> version.

I too want more justification of why this needs to be handled in the kernel[*].
The usefulness of this is dubious for many modern setups/workloads, and outright
undesirable for some, e.g. many (most?) cloud providers want to make it all but
impossible to access customer data.

[*] https://lore.kernel.org/all/ZSlNsn-f1j2bB8pW@FVFF77S0Q05N.cambridge.arm.com