On 06/05/21 20:42, Ben Gardon wrote:
In preparation for lazily allocating the rmaps when the TDP MMU is in
use, protect the rmaps with SRCU. Unfortunately, this requires
propagating a pointer to struct kvm around to several functions.
Thinking more about it, this is not needed because all reads of the rmap
array are guarded by the load-acquire of kvm->arch.memslots_have_rmaps.
That is, the pattern is always
if (!load-acquire(memslot_have_rmaps))
return;
... = __gfn_to_rmap(...)
slots->arch.rmap[x] = ...
store-release(memslot_have_rmaps, true)
where the load-acquire/store-release have the same role that
srcu_dereference/rcu_assign_pointer had before this patch.
We also know that any read that misses the check has the potential for a
NULL pointer dereference, so it *has* to be like that.
That said, srcu_dereference has zero cost unless debugging options are
enabled, and it *is* true that the rmap can disappear if kvm->srcu is
not held, so I lean towards keeping this change and just changing the
commit message like this:
---------
Currently, rmaps are always allocated and published together with a new
memslot, so the srcu_dereference for the memslots array already ensures
that the memory pointed to by slots->arch.rmap is zero at the time
slots->arch.rmap. However, they still need to be accessed in an SRCU
read-side critical section, as the whole memslot can be deleted outside
SRCU.
--------
Thanks,
Paolo
Suggested-by: Paolo Bonzini<pbonzini@xxxxxxxxxx>
Signed-off-by: Ben Gardon<bgardon@xxxxxxxxxx>
---
arch/x86/kvm/mmu/mmu.c | 57 +++++++++++++++++++++++++-----------------
arch/x86/kvm/x86.c | 6 ++---
2 files changed, 37 insertions(+), 26 deletions(-)
