linux-kernel - Re: [PATCH 7/9] KVM: SVM: Treat exit_code as an unsigned 64-bit value through all of KVM

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Message-ID: <20251114182921.GB1725668@liuwe-devbox-debian-v2.local>
Date: Fri, 14 Nov 2025 18:29:21 +0000
From: Wei Liu <wei.liu@...nel.org>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Michael Kelley <mhklinux@...look.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Vitaly Kuznetsov <vkuznets@...hat.com>,
	"K. Y. Srinivasan" <kys@...rosoft.com>,
	Haiyang Zhang <haiyangz@...rosoft.com>,
	Wei Liu <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>,
	Nuno Das Neves <nunodasneves@...ux.microsoft.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Jim Mattson <jmattson@...gle.com>,
	Yosry Ahmed <yosry.ahmed@...ux.dev>
Subject: Re: [PATCH 7/9] KVM: SVM: Treat exit_code as an unsigned 64-bit
 value through all of KVM

On Fri, Nov 14, 2025 at 07:22:41AM -0800, Sean Christopherson wrote:
> On Fri, Nov 14, 2025, Michael Kelley wrote:
> > From: Sean Christopherson <seanjc@...gle.com> Sent: Thursday, November 13, 2025 2:56 PM
> > > 
> > 
> > Adding Microsoft's Nuno Das Neves to the "To:" line since he
> > originated the work to keep the Linux headers such as hvgdk.h in
> > sync with the Windows counterparts from which they originate.
> 
> ...
> 
> > >  /* Exit code reserved for hypervisor/software use */
> > > -#define SVM_EXIT_SW				0xf0000000
> > > +#define SVM_EXIT_SW				0xf0000000ull
> > > 
> > > -#define SVM_EXIT_ERR           -1
> > > +#define SVM_EXIT_ERR           -1ull
> > > 
> > 
> > [snip]
> > 
> > > diff --git a/include/hyperv/hvgdk.h b/include/hyperv/hvgdk.h
> > > index dd6d4939ea29..56b695873a72 100644
> > > --- a/include/hyperv/hvgdk.h
> > > +++ b/include/hyperv/hvgdk.h
> > > @@ -281,7 +281,7 @@ struct hv_vmcb_enlightenments {
> > >  #define HV_VMCB_NESTED_ENLIGHTENMENTS		31
> > > 
> > >  /* Synthetic VM-Exit */
> > > -#define HV_SVM_EXITCODE_ENL			0xf0000000
> > > +#define HV_SVM_EXITCODE_ENL			0xf0000000u
> > 
> > Is there a reason for making this Hyper-V code just "u", while
> > making the SVM_VMGEXIT_* values "ull"? I don't think
> > "u" vs. "ull" shouldn't make any difference when assigning to a
> > u64, but the inconsistency piqued my interest ....
> 
> I hedged and went for a more "minimal" change because it isn't KVM code, and at
> the time because I thought the value isn't defined by the APM.  Though looking
> again at the APM, it does reserve that value for software
> 
>   F000_000h    Unused    Reserved for Host.
> 
> and I can't find anything in the TLFS.  Ah, my PDF copy is just stale, it's indeed
> defined as a synthetic exit.
> 
>   https://learn.microsoft.com/en-us/virtualization/hyper-v-on-windows/tlfs/nested-virtualization#synthetic-vm-exit
> 
> Anyways, I'm in favor of making HV_SVM_EXITCODE_ENL an ull, though part of me
> wonders if we should do:
> 
>   #define HV_SVM_EXITCODE_ENL	SVM_EXIT_SW

I know this is very tempting, but these headers are supposed to mirror
Microsoft's internal headers, so we would like to keep them
self-contained for ease of tracking.

It should be fine to add the "ull" suffix here. I briefly talked to a
hypervisor developer and they agreed.

Thanks,
Wei