Re: [PATCH v3 07/24] Documentation for memory tagging remote packets

Luis Machado via Gdb-patches
2020-11-17T12:01:48Z

Hi,

On 11/17/20 7:05 AM, David Spickett wrote:
>> Right now the design makes these types architecture-specific.
>
> This works too, in fact it matches the breakpoint types example better that way.
>
>> But there's one catch right now. The user-visible commands know about
>> two types of tags (logical and allocation). The native/remote side of
>> GDB only sees one type, the allocation one, as it doesn't make sense to
>> ask the native/remote target about logical tags.
>>
>> This is slightly messy and, in my opinion, should be an implementation
>> detail.
>
> Tell me if I have this right.
>
> In gdb in overall you have these two types but the server only uses
> one of them, the allocation tag type.
> So only the allocation tag type will ever go over the protocol. (for
> MTE at least)

That's correct. Only GDB knows about logical tags. Those don't make
their way to the remote via the remote protocol.

>
> Given that, if we assume that "mte allocation" type is 1. A future
> AArch64 kind of memory tagging could allocate 2 and on for its tag
> types.
> Something like:
> AArch64 Memory Tag types -
> 0 : MTE logical (which is internal only, reserved but documented as
> unused, or left out completely?)
> 1 : MTE allocation (the one we use at present)
> 2: <future tagging> logical tag (because maybe there is some server
> component for this kind of tagging extension?)
> 3: <future tagging> allocation tag
>
> The reason I want to clarify is that I understood the type to
> differentiate tagging technologies, not the kind of tag within them.
> (the type tells you MTE vs <future tag type> instead of allocation vs logical)
> The use case being what if you have MTE and <future tag type> active
> in the same target and I want to set an MTE allocation tag,
> how can the server tell them apart?

Right. The type is really telling you what specific kind of tag you are
requesting, not the technology. So it may be perfectly valid to request
a MTE logical tag from the remote target, but the remote target doesn't
know how to reply to that at the moment (nor does it make much sense, IMO).

The tag types don't overlap at the moment, given they are an ENUM in
generic code. So the server will tell them apart by their values.

>
> If the type numbers overlap between tagging technologies, we can't
> tell them apart.
> However if they encode what extension they are for and the
> logical/allocation type (as in the example above) then we can.
>
> A lot of that is probably academic given there's one relevant type but
> we can at least document the intent of the field.
> E.g. "these types are global to AArch64 so new types should not
> overlap existing ones"

I suppose. I chose to have generic ENUM's without specific references to
MTE for that reason. Architectures can use logical/allocation tags as
they see fit, but the ENUM values will not overlap.

We need to support the UI as well, so there needs to be some generic
definitions so commands can query different tag types.

>
>> Otherwise we'd need to standardize on particular tag type names across
>> different architectures, like "hw memory tag", "sw memory tag",
>> "capability tag" etc.
>
> Well I was thinking of type more as a single value like "mte". Anyway
> I'm fine with the integer route.

Though we don't have a use for requesting logical tags from the remote
targets, it is possible to support that. Passing "mte" or any other
technology name would close that option.

If, for example, we decide to have a dumb GDB client and a smart
GDBserver (unlikely at this point), then it would make sense to pass
down logical tag requests I think. I just want to keep that option open
if someone wants to do it, or if some other type of tag shows up that
would require such support.

>
> On Mon, 16 Nov 2020 at 17:23, Luis Machado <luis.machado@linaro.org> wrote:
>>
>>
>>
>> On 11/16/20 1:04 PM, David Spickett wrote:
>>> Also with regard to the "type" field.
>>>
>>>> +@var{type} is the type of tag the request wants to fetch. The typeis a signed
>>>> +integer.
>>>
>>> (typo aside) Is this field architecture specific and will there be a
>>> list of these type numbers documented anywhere? (or already is)
>>> For example would 1 on AArch64 be MTE, and on <other arch> be <other
>>> tag type>. Or would that <other tag type> be 2.
>>>
>>> My assumption has been that it is the latter and that a value means a
>>> kind of tagging extension. So for example 1=MTE rather than
>>> 1= mte logical and 2 = mte allocation. Correct me if I am wrong there.
>>
>> Right now the design makes these types architecture-specific. It would
>> be nice to have more documentation about them, for sure.
>>
>> But there's one catch right now. The user-visible commands know about
>> two types of tags (logical and allocation). The native/remote side of
>> GDB only sees one type, the allocation one, as it doesn't make sense to
>> ask the native/remote target about logical tags.
>>
>> This is slightly messy and, in my opinion, should be an implementation
>> detail.
>>
>> So, in summary... We have a couple generic tag types GDB knows about:
>> logical and allocation.
>>
>> Those types get translated to an arch/a target-specific type when they
>> cross the native/remote target boundary.
>>
>> In theory we could have generic tag types 1 and 2 in generic code, but
>> tag type 2 gets translated to type 1 in a remote packet.
>>
>> Maybe we could improve this a little.
>>
>>>
>>> A page like:
>>> https://sourceware.org/gdb/current/onlinedocs/gdb/ARM-Breakpoint-Kinds.html#ARM-Breakpoint-Kinds
>>>
>>> Or just a short note, given that there's only one type right now.
>>
>> Yes, that would be nice to expand for the tag types.
>>
>>>
>>> Also, I may have suggested the type be a string at some point. However
>>> based on examples like the link above
>>> I don't see much advantage to it apart from making packet dumps easier
>>> to read. Just wanted to close the loop on that
>>> if I didn't before.
>>
>> I don't have a strong preference here. I'm just forwarding the tag type
>> from generic code.
>>
>> If we want to pass strings, we will need a gdbarch hook that maps a type
>> to a string in the remote target layer.
>>
>> Otherwise we'd need to standardize on particular tag type names across
>> different architectures, like "hw memory tag", "sw memory tag",
>> "capability tag" etc.
>>
>>>
>>>
>>>
>>> On Mon, 16 Nov 2020 at 15:44, David Spickett <david.spickett@linaro.org> wrote:
>>>>
>>>> Minor thing, there is a missing space here in "typeis".
>>>>
>>>>> +@var{type} is the type of tag the request wants to fetch. The typeis a signed
>>>>> +integer.
>>>>
>>>> On Mon, 9 Nov 2020 at 17:08, Eli Zaretskii <eliz@gnu.org> wrote:
>>>>>
>>>>>> Date: Mon, 9 Nov 2020 14:04:18 -0300
>>>>>> From: Luis Machado via Gdb-patches <gdb-patches@sourceware.org>
>>>>>> Cc: david.spickett@linaro.org
>>>>>>
>>>>>> gdb/doc/ChangeLog:
>>>>>>
>>>>>> YYYY-MM-DD Luis Machado <luis.machado@linaro.org>
>>>>>>
>>>>>> * gdb.texinfo (General Query Packets): Document qMemTags and
>>>>>> QMemTags. Document the "memory-tagging" feature.
>>>>>> ---
>>>>>> gdb/doc/gdb.texinfo | 96 +++++++++++++++++++++++++++++++++++++++++++++
>>>>>> 1 file changed, 96 insertions(+)
>>>>>
>>>>> OK for this part, thanks.