KVM: MMU: document mmu-lock and fast page fault

(to be written)

2. Reference

2: Exception

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Fast page fault:

Fast page fault is the fast path which fixes the guest page fault out of

the mmu-lock on x86. Currently, the page fault can be fast only if the

shadow page table is present and it is caused by write-protect, that means

we just need change the W bit of the spte.

What we use to avoid all the race is the SPTE_HOST_WRITEABLE bit and

SPTE_MMU_WRITEABLE bit on the spte:

- SPTE_HOST_WRITEABLE means the gfn is writable on host.

- SPTE_MMU_WRITEABLE means the gfn is writable on mmu. The bit is set when

  the gfn is writable on guest mmu and it is not write-protected by shadow

  page write-protection.

On fast page fault path, we will use cmpxchg to atomically set the spte W

bit if spte.SPTE_HOST_WRITEABLE = 1 and spte.SPTE_WRITE_PROTECT = 1, this

is safe because whenever changing these bits can be detected by cmpxchg.

But we need carefully check these cases:

1): The mapping from gfn to pfn

The mapping from gfn to pfn may be changed since we can only ensure the pfn

is not changed during cmpxchg. This is a ABA problem, for example, below case

will happen:

At the beginning:

gpte = gfn1

gfn1 is mapped to pfn1 on host

spte is the shadow page table entry corresponding with gpte and

spte = pfn1

   VCPU 0                           VCPU0

on fast page fault path:

   old_spte = *spte;

                                 pfn1 is swapped out:

                                    spte = 0;

                                 pfn1 is re-alloced for gfn2.

                                 gpte is changed to point to

                                 gfn2 by the guest:

                                    spte = pfn1;

   if (cmpxchg(spte, old_spte, old_spte+W)

	mark_page_dirty(vcpu->kvm, gfn1)

             OOPS!!!

We dirty-log for gfn1, that means gfn2 is lost in dirty-bitmap.

For direct sp, we can easily avoid it since the spte of direct sp is fixed

to gfn. For indirect sp, before we do cmpxchg, we call gfn_to_pfn_atomic()

to pin gfn to pfn, because after gfn_to_pfn_atomic():

- We have held the refcount of pfn that means the pfn can not be freed and

  be reused for another gfn.

- The pfn is writable that means it can not be shared between different gfns

  by KSM.

Then, we can ensure the dirty bitmaps is correctly set for a gfn.

Currently, to simplify the whole things, we disable fast page fault for

indirect shadow page.

2): Dirty bit tracking

In the origin code, the spte can be fast updated (non-atomically) if the

spte is read-only and the Accessed bit has already been set since the

Accessed bit and Dirty bit can not be lost.

But it is not true after fast page fault since the spte can be marked

writable between reading spte and updating spte. Like below case:

At the beginning:

spte.W = 0

spte.Accessed = 1

   VCPU 0                                       VCPU0

In mmu_spte_clear_track_bits():

   old_spte = *spte;

   /* 'if' condition is satisfied. */

   if (old_spte.Accssed == 1 &&

        old_spte.W == 0)

      spte = 0ull;

                                         on fast page fault path:

                                             spte.W = 1

                                         memory write on the spte:

                                             spte.Dirty = 1

   else

      old_spte = xchg(spte, 0ull)

   if (old_spte.Accssed == 1)

      kvm_set_pfn_accessed(spte.pfn);

   if (old_spte.Dirty == 1)

      kvm_set_pfn_dirty(spte.pfn);

      OOPS!!!

The Dirty bit is lost in this case.

In order to avoid this kind of issue, we always treat the spte as "volatile"

if it can be updated out of mmu-lock, see spte_has_volatile_bits(), it means,

the spte is always atomicly updated in this case.

3): flush tlbs due to spte updated

If the spte is updated from writable to readonly, we should flush all TLBs,

otherwise rmap_write_protect will find a read-only spte, even though the

writable spte might be cached on a CPU's TLB.

As mentioned before, the spte can be updated to writable out of mmu-lock on

fast page fault path, in order to easily audit the path, we see if TLBs need

be flushed caused by this reason in mmu_spte_update() since this is a common

function to update spte (present -> present).

Since the spte is "volatile" if it can be updated out of mmu-lock, we always

atomicly update the spte, the race caused by fast page fault can be avoided,

See the comments in spte_has_volatile_bits() and mmu_spte_update().

3. Reference

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Name:		kvm_lock

Protects:	- kvm_arch::{last_tsc_write,last_tsc_nsec,last_tsc_offset}

		- tsc offset in vmcb

Comment:	'raw' because updating the tsc offsets must not be preempted.

Name:		kvm->mmu_lock

Type:		spinlock_t

Arch:		any

Protects:	-shadow page/shadow tlb entry

Comment:	it is a spinlock since it is used in mmu notifier.