x86/mm: Use/Fix PCID to optimize user/kernel switches

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 <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Boris Ostrovsky <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Brian Gerst <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: David Laight <[email protected]>
Cc: Denys Vlasenko <[email protected]>
Cc: Eduardo Valentin <[email protected]>
Cc: Greg KH <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Josh Poimboeuf <[email protected]>
Cc: Juergen Gross <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Signed-off-by: Ingo Molnar <[email protected]>

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