git: e0612ed490c7 - main - amd64 pmap: add comment explaining why INVLPG is functional for PCID config

The branch main has been updated by kib:

URL: https://cgit.FreeBSD.org/src/commit/?id=e0612ed490c723174607823378d9e883a4b1c257

commit e0612ed490c723174607823378d9e883a4b1c257
Author:     Konstantin Belousov <kib@FreeBSD.org>
AuthorDate: 2022-10-10 00:19:45 +0000
Commit:     Konstantin Belousov <kib@FreeBSD.org>
CommitDate: 2022-10-10 21:33:17 +0000

    amd64 pmap: add comment explaining why INVLPG is functional for PCID config
    
    Reviewed by:    alc, markj
    Sponsored by:   The FreeBSD Foundation
    MFC after:      1 week
    Differential revision:  https://reviews.freebsd.org/D36919
---
 sys/amd64/amd64/pmap.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/sys/amd64/amd64/pmap.c b/sys/amd64/amd64/pmap.c
index f69f452bcced..2dddc7a4c582 100644
--- a/sys/amd64/amd64/pmap.c
+++ b/sys/amd64/amd64/pmap.c
@@ -2897,8 +2897,16 @@ pmap_update_pde_invalidate(pmap_t pmap, vm_offset_t va, pd_entry_t newpde)
  *   page table, and INVPCID(INVPCID_CTXGLOB)/invltlb_glob() for a
  *   user space page table(s).
  *
- *   If the INVPCID instruction is available, it is used to flush entries
- *   from the kernel page table.
+ *   If the INVPCID instruction is available, it is used to flush user
+ *   entries from the kernel page table.
+ *
+ *   When PCID is enabled, the INVLPG instruction invalidates all TLB
+ *   entries for the given page that either match the current PCID or
+ *   are global. Since TLB entries for the same page under different
+ *   PCIDs are unaffected, kernel pages which reside in all address
+ *   spaces could be problematic.  We avoid the problem by creating
+ *   all kernel PTEs with the global flag (PG_G) set, when PTI is
+ *   disabled.
  *
  * * mode: PTI disabled, PCID present.  The kernel reserves PCID 0 for its
  *   address space, all other 4095 PCIDs are used for user mode spaces