On Wed Apr 29, 2020 at 7:48 AM, Christophe Leroy wrote:

Le 29/04/2020 à 04:05, Christopher M. Riedl a écrit :

quoted

x86 supports the notion of a temporary mm which restricts access to
temporary PTEs to a single CPU. A temporary mm is useful for situations
where a CPU needs to perform sensitive operations (such as patching a
STRICT_KERNEL_RWX kernel) requiring temporary mappings without exposing
said mappings to other CPUs. A side benefit is that other CPU TLBs do
not need to be flushed when the temporary mm is torn down.

Mappings in the temporary mm can be set in the userspace portion of the
address-space.

Interrupts must be disabled while the temporary mm is in use. HW
breakpoints, which may have been set by userspace as watchpoints on
addresses now within the temporary mm, are saved and disabled when
loading the temporary mm. The HW breakpoints are restored when unloading
the temporary mm. All HW breakpoints are indiscriminately disabled while
the temporary mm is in use.

Why do we need to use a temporary mm all the time ?

Not sure I understand, the temporary mm is only in use for kernel
patching in this series. We could have other uses in the future maybe
where it's beneficial to keep mappings local.

Doesn't each CPU have its own mm already ? Only the upper address space
is shared between all mm's but each mm has its own lower address space,
at least when it is running a user process. Why not just use that mm ?
As we are mapping then unmapping with interrupts disabled, there is no
risk at all that the user starts running while the patch page is mapped,
so I'm not sure why switching to a temporary mm is needed.

I suppose that's an option, but then we have to save and restore the
mapping which we temporarily "steal" from userspace. I admit I didn't
consider that as an option when I started this series based on the x86
patches. I think it's cleaner to switch mm, but that's a rather weak
argument. Are you concerned about performance with the temporary mm?

quoted

Based on x86 implementation:

commit cefa929c034e
("x86/mm: Introduce temporary mm structs")

Signed-off-by: Christopher M. Riedl <redacted>

Christophe