RE: [PATCH v2 1/3] gdb: Make tagged pointer support configurable.

["Schimpe, Christina" <christina.schimpe@intel.com>]

Hi Luis, 

Do you still have concerns on the topic discussed below?
Otherwise, I'd apply Felix latest feedback and push a new version of this series.

Thanks,
Christina

> -----Original Message-----
> From: Schimpe, Christina
> Sent: Monday, June 10, 2024 5:06 PM
> To: Luis Machado <luis.machado@arm.com>; Willgerodt, Felix
> <FeliX.Willgerodt@intel.com>; gdb-patches@sourceware.org
> Cc: eliz@gnu.org
> Subject: RE: [PATCH v2 1/3] gdb: Make tagged pointer support configurable.
> 
> 
> 
> > -----Original Message-----
> > From: Luis Machado <luis.machado@arm.com>
> > Sent: Monday, June 10, 2024 4:01 PM
> > To: Schimpe, Christina <christina.schimpe@intel.com>; Willgerodt,
> > Felix <felix.willgerodt@intel.com>; gdb-patches@sourceware.org
> > Cc: eliz@gnu.org
> > Subject: Re: [PATCH v2 1/3] gdb: Make tagged pointer support configurable.
> >
> > On 6/3/24 15:13, Schimpe, Christina wrote:
> > > Hi Luis,
> > >
> > >>>>> -/* AArch64 implementation of the remove_non_address_bits
> > >>>>> gdbarch
> > >>>> hook.
> > >>>>> Remove
> > >>>>> -   non address bits from a pointer value.  */
> > >>>>> -
> > >>>>> -static CORE_ADDR
> > >>>>> +CORE_ADDR
> > >>>>>  aarch64_remove_non_address_bits (struct gdbarch *gdbarch,
> > >> CORE_ADDR
> > >>>>> pointer)
> > >>>>
> > >>>> Shouldn't there still be some sort of comment for this function
> > >>>> in the c
> > >> file?
> > >>>> At least some "see header file"? Though it seems like no function
> > >>>> in
> > >>>> aarch64- tdep.h has any comment, and all comments are in
> > >>>> aarch64-tdep.c. I would also be fine with that for consistency.
> > >>>> Maybe Luis can comment how he prefers it.
> > >>>>
> > >>>> But I am fine with this patch. Let's see if someone else objects this
> split.
> > >>>>
> > >>>> Reviewed-By: Felix Willgerodt <felix.willgerodt@intel.com>
> > >>>>
> > >>>> Thanks,
> > >>>> Felix
> > >>>
> > >>> Ok, let's wait for further comments here.
> > >>
> > >> Sorry, only spotted this now.
> > >>
> > >> The comment on the above function would be a repetition of the hook
> > >> explanation, and the gdbarch hook explanation covers all the
> > >> details we need. But if the comment is being removed due to it
> > >> being in the header, we should point to the header instead.
> > >>
> > >> "See aarch64-tdep.h" should do.
> > >
> > > Ok, I will add that line.
> > >
> > >> There is something odd about this patch though. The aarch64 code is
> > >> being changed to call aarch64_remove_non_address_bits directly, but
> > >> 3 new hooks are being set:
> > >>
> > >> +  set_gdbarch_remove_non_address_bits_watchpoint
> > >> +    (gdbarch, aarch64_remove_non_address_bits);
> > >> + set_gdbarch_remove_non_address_bits_breakpoint
> > >> +    (gdbarch, aarch64_remove_non_address_bits);
> > >> + set_gdbarch_remove_non_address_bits_memory
> > >> +    (gdbarch, aarch64_remove_non_address_bits);
> > >>
> > >> But the above hooks (the gdbarch_remove_non_address_bits_memory
> at
> > >> least) never get used in aarch64 code. Is there a reason for that?
> > >
> > > I thought it's wrong to call the
> > > gdbarch_remove_non_address_bits_memory
> > hook in aarch64 code.
> > > The reason is described here:
> > > https://sourceware.org/pipermail/gdb-patches/2024-May/209420.html
> > >
> > > "Also I noticed that the gdbarch function
> > 'gdbarch_remove_non_addr_bits_memory'
> > > should not be used in several functions of aarch64* files (patch 1)
> > > due to the following:
> > >
> > > The function description for 'gdbarch_remove_non_addr_bits_memory'
> > states:
> > >
> > > "Given a pointer for the architecture, this hook removes all the
> > > non-significant bits and sign-extends things as needed. It gets used
> > > to remove non-address bits from any pointer used to access memory. "
> > >
> > > We don't know if the pointer passed to
> > 'gdbarch_remove_non_addr_bits_memory'
> > > by 'gdbarch_tagged_address_p' will be used to access memory.  That's
> > > why it's wrong to call this function in 'gdbarch_tagged_address_p'.
> > >
> > > There are several similar wrong calls of
> > 'gdbarch_remove_non_addr_bits_memory'
> > > in the changes of patch 1.  I replaced all of them with
> > > 'aarch64_remove_non_address_bits'."
> > >
> >
> > That's fine, but previously the hook was correct for what is was being
> > used for in aarch64. This particular change means we no longer have a
> > hook that
> > aarch64 can use for the same purposes it used before. This part feels a bit
> odd.
> > It feels like the memory-specific one would be appropriate, but it isn't clear.
> 
> Hm, I thought it is not a problem to remove the hook in the aarch64 files, as
> it's target specific code.
> 
> > Also, from the 1/3 patch submission:
> >
> > "- In contrast to the implementation for ARM's TBI, the Linux kernel
> supports
> >   tagged pointers for memory access."
> >
> > Is there a negative impact if we feed the kernel an untagged pointer
> > for a particular memory access? If not, it sounds like it might be
> > simpler (from gdb's
> > perspective) to let gdb always unmask things, no? Of course, there
> > might be a performance impact of checking if a region is tagged.
> > I'm just trying to understand if the additional hooks are really
> needed/useful.
> >
> > Right now it looks as if we could make it work with the existing hook
> > in place, instead of splitting it into 3 different hooks.
> 
> I think in case of amd64 there is a negative impact.  In contrast to ARM's TBI,
> LAM can be enabled at runtime.  So we would have to parse the
> /proc/<pid>/status file every time when the pointer is used to access
> memory. With the 3 hooks I wanted to avoid this.
> 
> I tried to make the purpose more clear by adding the two sentences in the
> commit message for v2:
> 
> "This way, one can make sure that addresses are only adjusted when
> necessary.
> In case of LAM, this avoids unnecessary parsing of the /proc/<pid>/status file
> to get the untag mask."
> 
> So yes, it does work with the existing hook, but when it comes to
> performance, I think 3 hooks would be better.
> 
> Christina
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