Re: [PATCH] KVM/x86: don't use a literal 1 instead of RET_PF_RETRY

From: Jürgen Groß
Date: Sat Nov 09 2024 - 02:06:40 EST

On 08.11.24 23:12, Sean Christopherson wrote:
On Fri, Nov 08, 2024, Jürgen Groß wrote:
On 08.11.24 19:44, Sean Christopherson wrote:
On Fri, Nov 08, 2024, Paolo Bonzini wrote:
Queued, thanks.

Noooo! Can you un-queue?

The return from kvm_mmu_page_fault() is NOT RET_PF_xxx, it's KVM outer 0/1/-errno.
I.e. '1' is saying "resume the guest", it has *nothing* to do with RET_PF_RETRY.
E.g. that path also handles RET_PF_FIXED, RET_PF_SPURIOUS, etc.

And what about the existing "return RET_PF_RETRY" further up?

Oof. Works by coincidence. The intent in that case is to retry the fault, but
the fact that RET_PF_RETRY happens to be '1' is mostly luck. Returning a postive
value other than '1' should work, but as called out by the comments for the enum,
using '0' for CONTINUE isn't a hard requirement. E.g. if for some reason we used
'0' for RET_PF_RETRY, this code would break.

I think this function is an especially awful case, as it seems to be natural
to return a RET_PF_ value from a function named kvm_mmu_page_fault().


* Note, all values must be greater than or equal to zero so as not to encroach
* on -errno return values. Somewhat arbitrarily use '0' for CONTINUE, which
* will allow for efficient machine code when checking for CONTINUE, e.g.
* "TEST %rax, %rax, JNZ", as all "stop!" values are non-zero.

FWIW, you are far from the first person to complain about KVM's mostly-undocumented
0/1/-errno return encoding scheme. The problems is that it's so pervasive
throughout KVM, that in some cases it's not easy to understand if a function is
actually using that scheme, or just happens to return similar values. I.e.
converting to enums (or #defines) would require a lot of work and churn.

I think it would be helpful to at least add comments to the functions
returning the 0/1/-errno value.

And it would be even better to have #defines for the 0 and 1. New use cases
should use the #defines, and whether we convert current users is another
question (I'd go for it, as it is only a minor additional work when adding
the comments anyway).

If you are fine with that, I can start the effort.


Juergen

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