• Peter Zijlstra's avatar
    x86/mm: Use/Fix PCID to optimize user/kernel switches · 6fd166aa
    Peter Zijlstra authored
    We can use PCID to retain the TLBs across CR3 switches; including those now
    part of the user/kernel switch. This increases performance of kernel
    entry/exit at the cost of more expensive/complicated TLB flushing.
    
    Now that we have two address spaces, one for kernel and one for user space,
    we need two PCIDs per mm. We use the top PCID bit to indicate a user PCID
    (just like we use the PFN LSB for the PGD). Since we do TLB invalidation
    from kernel space, the existing code will only invalidate the kernel PCID,
    we augment that by marking the corresponding user PCID invalid, and upon
    switching back to userspace, use a flushing CR3 write for the switch.
    
    In order to access the user_pcid_flush_mask we use PER_CPU storage, which
    means the previously established SWAPGS vs CR3 ordering is now mandatory
    and required.
    
    Having to do this memory access does require additional registers, most
    sites have a functioning stack and we can spill one (RAX), sites without
    functional stack need to otherwise provide the second scratch register.
    
    Note: PCID is generally available on Intel Sandybridge and later CPUs.
    Note: Up until this point TLB flushing was broken in this series.
    
    Based-on-code-from: Dave Hansen <dave.hansen@linux.intel.com>
    Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
    Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
    Cc: Andy Lutomirski <luto@kernel.org>
    Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
    Cc: Borislav Petkov <bp@alien8.de>
    Cc: Brian Gerst <brgerst@gmail.com>
    Cc: Dave Hansen <dave.hansen@linux.intel.com>
    Cc: David Laight <David.Laight@aculab.com>
    Cc: Denys Vlasenko <dvlasenk@redhat.com>
    Cc: Eduardo Valentin <eduval@amazon.com>
    Cc: Greg KH <gregkh@linuxfoundation.org>
    Cc: H. Peter Anvin <hpa@zytor.com>
    Cc: Josh Poimboeuf <jpoimboe@redhat.com>
    Cc: Juergen Gross <jgross@suse.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Will Deacon <will.deacon@arm.com>
    Cc: aliguori@amazon.com
    Cc: daniel.gruss@iaik.tugraz.at
    Cc: hughd@google.com
    Cc: keescook@google.com
    Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
    6fd166aa
tlbflush.h 13 KB