Re: [PATCH v4 3/6] target/riscv: Add helper functions to calculate current number of masked bits for pointer masking

[Deepak Gupta]

On Sat, Jan 20, 2024 at 11:14 PM Alexey Baturo <baturo.alexey@gmail.com> wrote:
>
> Hi,
>
> Having the feature to run binaries with pointer masking on qemu-user is 
> really nice, but I see this patch series as an initial support.
> Obviously there'll be more patches and fixes for pointer masking as soon as 
> arch tests are ready.
> I suggest supporting qemu-user in the next patches, but make sure we do this 
> before claiming 100% support for pointer masking.
> @Deepak Gupta what do you think?

I think that makes sense. Thanks.

>
> Thanks
>
> сб, 20 янв. 2024 г. в 10:37, Richard Henderson <richard.henderson@linaro.org>:
>>
>> On 1/19/24 09:40, Deepak Gupta wrote:
>> > On Thu, Jan 18, 2024 at 12:50 PM Richard Henderson
>> > <richard.henderson@linaro.org> wrote:
>> >> At some point pointer masking will be in hardware, and the kernel will 
>> >> gain support for
>> >> it, and there will likely be a prctl() added for it.  At the point the 
>> >> kernel finalizes
>> >> the API, you will be able to enable pointer masking for qemu-user.
>> >
>> > I am sure I am missing some important detail here, BUT...
>> >
>> > How is it different from aarch64 "top byte ignore".
>>
>> It is very similar, yes.
>>
>> > I think commit: 16c8497 enables top byte ignore for user pointers and
>> > by default for qemu-user for aarch64 target.
>>
>> Not quite, no.
>>
>> commit 0e0c030c681730f3ec55ba3b223b608a8f3e8282
>> Author: Richard Henderson <richard.henderson@linaro.org>
>> Date:   Fri Feb 12 10:48:51 2021 -0800
>>
>>      linux-user/aarch64: Implement PR_TAGGED_ADDR_ENABLE
>>
>> is more relevant.
>>
>> > IIRC, user <--> kernel abi is only needed for pointers that are passed
>> > to the kernel.
>>
>> It is also needed to *enable* pointer masking at all.
>>
>> For aarch64, TBI has been enabled for user-space since the beginning, but 
>> that is not true
>> for riscv.  Therefore there will be a need for a syscall to opt in and 
>> enable pointer masking.
>>
>> > And in the case of qemu-user, we are talking about the host kernel.
>>
>> No, we are not.  We are always emulating the guest kernel.
>>
>>
>> r~