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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