Re: [PATCH v7 3/7] LoongArch: KVM: Add cpucfg area for kvm hypervisor

[maobibo]

On 2024/4/2 上午10:49, Xi Ruoyao wrote:
> On Tue, 2024-04-02 at 09:43 +0800, maobibo wrote:
> > > Sorry for the late reply, but I think it may be a bit non-constructive
> > > to repeatedly submit the same code without due explanation in our
> > > previous review threads. Let me try to recollect some of the details
> > > though...
> > Because your review comments about hypercall method is wrong, I need not
> > adopt it.
>
> Again it's unfair to say so considering the lack of LVZ documentation.
>
> /* snip */
>
> > 1. T0-T7 are scratch registers during SYSCALL ABI, this is what you
> > suggest, does there exist information leaking to user space from T0-T7
> > registers?
>
> It's not a problem.  When syscall returns RESTORE_ALL_AND_RET is invoked
> despite T0-T7 are not saved.  So a "junk" value will be read from the
> leading PT_SIZE bytes of the kernel stack for this thread.
For you it is "junk" value, some guys maybe thinks it is useful.

There is another issue, since kernel restore T0-T7 registers and user 
space save T0-T7. Why T0-T7 is scratch registers rather than preserve 
registers like other architecture? What is the advantage if it is 
scratch registers?

Regards
Bibo Mao
> The leading PT_SIZE bytes of the kernel stack is dedicated for storing
> the struct pt_regs representing the reg file of the thread in the
> userspace.
>
> Thus we may only read out the userspace T0-T7 value stored when the same
> thread was interrupted or trapped last time, or 0 (if the thread was
> never interrupted or trapped before).
>
> And it's impossible to read some data used by the kernel internally, or
> some data of another thread.
>
> But indeed there is some improvement here.  Zeroing these registers
> seems cleaner than reading out the junk values, and also faster (move
> $t0, $r0 is faster than ld.d $t0, $sp, PT_R12).  Not sure if it's worthy
> to violate Huacai's "keep things simple" aspiration though.