[PATCH 1/4] powerpc/kvm: Move kvm_tmp into .text, shrink to 64K

[Michael Ellerman]

On Wed, 2019-09-11 at 11:57:43 UTC, Michael Ellerman wrote:
> In some configurations of KVM, guests binary patch themselves to
> avoid/reduce trapping into the hypervisor. For some instructions this
> requires replacing one instruction with a sequence of instructions.
> 
> For those cases we need to write the sequence of instructions
> somewhere and then patch the location of the original instruction to
> branch to the sequence. That requires that the location of the
> sequence be within 32MB of the original instruction.
> 
> The current solution for this is that we create a 1MB array in BSS,
> write sequences into there, and then free the remainder of the array.
> 
> This has a few problems:
>  - it confuses kmemleak.
>  - it confuses lockdep.
>  - it requires mapping kvm_tmp executable, which can cause adjacent
>    areas to also be mapped executable if we're using 16M pages for the
>    linear mapping.
>  - the 32MB limit can be exceeded if the kernel is big enough,
>    especially with STRICT_KERNEL_RWX enabled, which then prevents the
>    patching from working at all.
> 
> We can fix all those problems by making kvm_tmp just a region of
> regular .text. However currently it's 1MB in size, and we don't want
> to waste 1MB of text. In practice however I only see ~30KB of kvm_tmp
> being used even for an allyes_config. So shrink kvm_tmp to 64K, which
> ought to be enough for everyone, and move it into .text.
> 
> Signed-off-by: Michael Ellerman <mpe at ellerman.id.au>

Series applied to powerpc next.

https://git.kernel.org/powerpc/c/0cb0837f9db1a6ed5b764ef61dd5f1a314b8231a

cheers