Re: [PATCHv8 00/11] Linear Address Masking enabling

[Bharata B Rao]

Hi Kirill,

On 9/4/2022 6:30 AM, Kirill A. Shutemov wrote:
> On Tue, Aug 30, 2022 at 04:00:53AM +0300, Kirill A. Shutemov wrote:
>> Linear Address Masking[1] (LAM) modifies the checking that is applied to
>> 64-bit linear addresses, allowing software to use of the untranslated
>> address bits for metadata.
>>
>> The patchset brings support for LAM for userspace addresses. Only LAM_U57 at
>> this time.
>>
>> Please review and consider applying.
>>
>> git://git.kernel.org/pub/scm/linux/kernel/git/kas/linux.git lam
> 
> +Bharata, Ananth.
> 
> Do you folks have any feedback on the patchset?
> 
> Looks like AMD version of the tagged pointers feature does not get
> traction as of now, but I want to be sure that the interface introduced
> here can be suitable for your future plans.
> 
> Do you see anything in the interface that can prevent it to be extended to
> the AMD feature?

The arch_prctl() extensions is generic enough that it should be good.

The untagged_addr() macro looks like this from one of the callers:

        start = untagged_addr(mm, start);
ffffffff814d39bb:       48 8b 8d 40 ff ff ff    mov    -0xc0(%rbp),%rcx
ffffffff814d39c2:       48 89 f2                mov    %rsi,%rdx
ffffffff814d39c5:       48 c1 fa 3f             sar    $0x3f,%rdx
ffffffff814d39c9:       48 0b 91 50 03 00 00    or     0x350(%rcx),%rdx
ffffffff814d39d0:       48 21 f2                and    %rsi,%rdx
ffffffff814d39d3:       49 89 d6                mov    %rdx,%r14

Can this overhead of a few additional instructions be removed for
platforms that don't have LAM feature? I haven't measured how much
overhead this effectively contributes to in real, but wonder if it is
worth optimizing for non-LAM platforms.

Regards,
Bharata.