| Message ID | 20241108161312.28365-1-jgross@suse.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | KVM/x86: don't use a literal 1 instead of RET_PF_RETRY | expand |
Queued, thanks. Paolo
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.
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? Juergen
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. * 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.
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c index 8e853a5fc867..d4a9f845b373 100644 --- a/arch/x86/kvm/mmu/mmu.c +++ b/arch/x86/kvm/mmu/mmu.c @@ -6157,7 +6157,7 @@ int noinline kvm_mmu_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa, u64 err vcpu->stat.pf_spurious++; if (r != RET_PF_EMULATE) - return 1; + return RET_PF_RETRY; emulate: return x86_emulate_instruction(vcpu, cr2_or_gpa, emulation_type, insn,
Using a literal 1 instead of RET_PF_RETRY is not nice, fix that. Signed-off-by: Juergen Gross <jgross@suse.com> --- arch/x86/kvm/mmu/mmu.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)