Hello,

On 22/01/2019 03:11, Masami Hiramatsu wrote:

On Mon, 21 Jan 2019 17:04:01 +0000
James Morse [off-list ref] wrote:

quoted

On systems with VHE the kernel and KVM's world-switch code run at the
same exception level. Code that is only used on a VHE system does not
need to be annotated as __hyp_text as it can reside anywhere in the
kernel text.

__hyp_text was also used to prevent kprobes from patching breakpoint
instructions into this region, as this code runs at a different
exception level. While this is no longer true with VHE, KVM still
switches VBAR_EL1, meaning a kprobe's breakpoint executed in the
world-switch code will cause a hyp-panic.

quoted

Annotate the VHE world-switch functions that aren't marked
__hyp_text as __kprobes.

quoted

---
This has been an issue since the VHE/non-VHE world-switch paths were
split. Before then the code was common, and covered by __hyp_text, which
is always blacklisted by a subsequent patch.

Thank you very much for fixing it!

BTW, would you mind if I ask you using NOKPROBE_SYMBOL() macro instead of
__kprobes attribute? __kprobes moves the function into __kprobe_text
forcibly, OTOH, NOKPROBE_SYMBOL() has no such side-effect.

Aha, yes. __kprobes moves the function to a special section, whereas the macro
spits out the address of the function into the blacklist section, which is
processed via init_kprobes().
I used __kprobes as its in keeping with __hyp_text, but this is clearly better
as it doesn't restrict the layout of the code. (and it solves the
hibernate/kexec problems as those would otherwise need to be in two sections!)

For my own education, when should __kprobes be used? Is it legacy?


Thanks!

James

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