Re: [RFC] x86/alternatives: Merge first and second step in text_poke_bp_batch

[Jiri Olsa]

On Tue, Jan 14, 2025 at 02:38:42PM +0000, David Laight wrote:
> From: Jiri Olsa
> > Sent: 14 January 2025 14:03
> > 
> > hi,
> > while checking on similar code for uprobes I was wondering if we
> > can merge first 2 steps of instruction update in text_poke_bp_batch
> > function.
> > 
> > Basically the first step now would be to write int3 byte together
> > with the rest of the bytes of the new instruction instead of doing
> > that separately. And the second step would be to overwrite int3
> > byte with first byte of the new instruction.
> > 
> > Would that work or do I miss some x86 detail that could lead to crash?
> 
> I suspect it will 'crash and burn'.
> 
> Consider what happens if there is a cache-line boundary in the
> middle of an instruction.
> (Actually an instruction fetch boundary will do.)
> 
> cpu0: reads the old instructions from the old cache line.
> cpu0: pipeline busy (or similar) so doesn't read the next cache line.
> cpu1: writes the new instructions.
> cpu0: reads the second cache line.
> 
> cpu0 now has a mix of the old and new instruction bytes.
> 
> Writing the int3 is safe - provided they don't return until
> all the patching is over.
> 
> But between writing the int3 (over the first opcode byte) and
> updating anything else I suspect you need something that does
> a complete synchronise between the cpu that discards any bytes
> in the decode pipeline as well as flushing the I-cache (etc).
> I suspect that requires an acked IPI.
> 
> Very long cpu stalls are easy to generate.
> Any read from PCIe will be slow (I've at fpga target that takes ~1us).
> You'd need to be unlucky to be patching an instruction while one
> was pending, but a DMA access might just be enough to cause grief.

ok, thanks for all the details,

jirka