Re: [PATCH] x86/mm/pti: in pti_clone_pgtable() don't increase addr by PUD_SIZE

[Dave Hansen]

On 8/20/19 12:51 AM, Song Liu wrote:
> In our x86_64 kernel, pti_clone_pgtable() fails to clone 7 PMDs because
> of this issuse, including PMD for the irq entry table. For a memcache
> like workload, this introduces about 4.5x more iTLB-load and about 2.5x
> more iTLB-load-misses on a Skylake CPU.

I was surprised that this manifests as a performance issue.  Usually
messing up PTI page table manipulation means you get to experience the
jobs of debugging triple faults.  But, it makes sense if its this line:

        /*
         * Note that this will undo _some_ of the work that
         * pti_set_kernel_image_nonglobal() did to clear the
         * global bit.
         */
        pti_clone_pgtable(start, end_clone, PTI_LEVEL_KERNEL_IMAGE);

which is restoring the Global bit.

*But*, that shouldn't get hit on a Skylake CPU since those have PCIDs
and shouldn't have a global kernel image.  Could you confirm whether
PCIDs are supported on this CPU?

>  		pud = pud_offset(p4d, addr);
>  		if (pud_none(*pud)) {
> -			addr += PUD_SIZE;
> +			addr += PMD_SIZE;
>  			continue;
>  		}

Did we also bugger up this code:

                pmd = pmd_offset(pud, addr);
                if (pmd_none(*pmd)) {
                        addr += PMD_SIZE;
                        continue;
                }

if we're on 32-bit and this:

#define PTI_LEVEL_KERNEL_IMAGE  PTI_CLONE_PTE

and we get a hole walking to a non-PMD-aligned address?