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In short, pmap_enter_quick_locked("user space", ..., VM_PROT_READ) doesn't

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work.  More precisely, it doesn't set ATTR_AP(ATTR_AP_USER) in the page
table entry, so any attempt to read from the mapped page by user space
generates a page fault.  This problem has gone unnoticed because the page
fault handler, vm_fault(), will ultimately call pmap_enter(), which
replaces the non-working page table entry with one that has
ATTR_AP(ATTR_AP_USER) set.

This change reduces the number of page faults during a "buildworld" by
about 19.4%.

Reviewed by:	andrew, markj
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D21841
This commit is contained in:
alc 2019-10-01 15:33:47 +00:00
parent 4c4fb8b83e
commit b5c0c1bef3
1 changed files with 2 additions and 2 deletions
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4
sys/arm64/arm64/pmap.c
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@ -3752,8 +3752,8 @@ pmap_enter_quick_locked(pmap_t pmap, vm_offset_t va, vm_page_t m,
ATTR_AP(ATTR_AP_RO) | L3_PAGE;
if ((prot & VM_PROT_EXECUTE) == 0 || m->md.pv_memattr == DEVICE_MEMORY)
l3_val |= ATTR_XN;
else if (va < VM_MAXUSER_ADDRESS)
l3_val |= ATTR_PXN;
if (va < VM_MAXUSER_ADDRESS)
l3_val |= ATTR_AP(ATTR_AP_USER) | ATTR_PXN;
/*
* Now validate mapping with RO protection