Re: Patch "KVM: VMX: Convert vcpu_vmx.exit_reason to a union" has been added to the 5.10-stable tree

[Sean Christopherson <seanjc@google.com>]

+Paolo

On Sun, Apr 18, 2021, Sasha Levin wrote:
> This is a note to let you know that I've just added the patch titled
> 
>     KVM: VMX: Convert vcpu_vmx.exit_reason to a union
> 
> to the 5.10-stable tree which can be found at:
>     http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
> 
> The filename of the patch is:
>      kvm-vmx-convert-vcpu_vmx.exit_reason-to-a-union.patch
> and it can be found in the queue-5.10 subdirectory.
> 
> If you, or anyone else, feels it should not be added to the stable tree,
> please let <stable@vger.kernel.org> know about it.

I'm not sure we want this going into stable kernels, even for 5.10 and 5.11.
I assume it got pulled in to resolve a conflict with commit 04c4f2ee3f68 ("KVM:
VMX: Don't use vcpu->run->internal.ndata as an array index"), but that's should
be trivial to resolve since it's just a collision with surrounding code.

Maybe we'll end up with a more painful conflict in the future that would be best
solved by grabbing this refactoring, but I don't think we're there yet.


> commit 1499b54db7d9e7e5f5014307e8391e3ad7986f1f
> Author: Sean Christopherson <seanjc@google.com>
> Date:   Fri Nov 6 17:03:12 2020 +0800
> 
>     KVM: VMX: Convert vcpu_vmx.exit_reason to a union
>     
>     [ Upstream commit 8e53324021645f820a01bf8aa745711c802c8542 ]
>     
>     Convert vcpu_vmx.exit_reason from a u32 to a union (of size u32).  The
>     full VM_EXIT_REASON field is comprised of a 16-bit basic exit reason in
>     bits 15:0, and single-bit modifiers in bits 31:16.
>     
>     Historically, KVM has only had to worry about handling the "failed
>     VM-Entry" modifier, which could only be set in very specific flows and
>     required dedicated handling.  I.e. manually stripping the FAILED_VMENTRY
>     bit was a somewhat viable approach.  But even with only a single bit to
>     worry about, KVM has had several bugs related to comparing a basic exit
>     reason against the full exit reason store in vcpu_vmx.
>     
>     Upcoming Intel features, e.g. SGX, will add new modifier bits that can
>     be set on more or less any VM-Exit, as opposed to the significantly more
>     restricted FAILED_VMENTRY, i.e. correctly handling everything in one-off
>     flows isn't scalable.  Tracking exit reason in a union forces code to
>     explicitly choose between consuming the full exit reason and the basic
>     exit, and is a convenient way to document and access the modifiers.
>     
>     No functional change intended.
>     
>     Cc: Xiaoyao Li <xiaoyao.li@intel.com>
>     Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
>     Signed-off-by: Chenyi Qiang <chenyi.qiang@intel.com>
>     Message-Id: <20201106090315.18606-2-chenyi.qiang@intel.com>
>     Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
>     Signed-off-by: Sasha Levin <sashal@kernel.org>