linux-kernel - Re: [PATCH v10 14/39] KVM: nSVM: Keep track of Hyper-V hv_vm_id/hv_vp

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Message-ID: <Yyt/Nrh4aoLrNt11@google.com>
Date:   Wed, 21 Sep 2022 21:16:38 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Michael Kelley <mikelley@...rosoft.com>,
        Siddharth Chandrasekaran <sidcha@...zon.de>,
        Yuan Yao <yuan.yao@...ux.intel.com>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v10 14/39] KVM: nSVM: Keep track of Hyper-V
 hv_vm_id/hv_vp_id

On Wed, Sep 21, 2022, Vitaly Kuznetsov wrote:
> Similar to nSVM, KVM needs to know L2's VM_ID/VP_ID and Partition
> assist page address to handle L2 TLB flush requests.
> 
> Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> ---
>  arch/x86/kvm/svm/hyperv.h | 16 ++++++++++++++++
>  arch/x86/kvm/svm/nested.c |  2 ++
>  2 files changed, 18 insertions(+)
> 
> diff --git a/arch/x86/kvm/svm/hyperv.h b/arch/x86/kvm/svm/hyperv.h
> index 7d6d97968fb9..8cf702fed7e5 100644
> --- a/arch/x86/kvm/svm/hyperv.h
> +++ b/arch/x86/kvm/svm/hyperv.h
> @@ -9,6 +9,7 @@
>  #include <asm/mshyperv.h>
>  
>  #include "../hyperv.h"
> +#include "svm.h"
>  
>  /*
>   * Hyper-V uses the software reserved 32 bytes in VMCB
> @@ -32,4 +33,19 @@ struct hv_enlightenments {
>   */
>  #define VMCB_HV_NESTED_ENLIGHTENMENTS VMCB_SW
>  
> +static inline void nested_svm_hv_update_vm_vp_ids(struct kvm_vcpu *vcpu)
> +{
> +	struct vcpu_svm *svm = to_svm(vcpu);
> +	struct hv_enlightenments *hve =
> +		(struct hv_enlightenments *)svm->nested.ctl.reserved_sw;

Eww :-)

I posted a small series to fix the casting[*], and as noted in the cover letter it's
going to conflict mightily.  Ignoring merge order for the moment, looking at the
series as a whole, if the Hyper-V definitions are moved to hyperv-tlfs.h, then I'm
tempted to say there's no need for svm/hyperv.h.

There should never be users of this stuff outside of svm/nested.c, and IMO there's
not enough stuff to warrant a separate set of files.  nested_svm_hv_update_vp_assist()
isn't SVM specific and fits better alongside kvm_hv_get_assist_page().

That leaves three functions and ~40 lines of code, which can easily go directly
into svm/nested.c.

I'm definitely not dead set against having hyperv.{ch}, but unless there's a high
probability of SVM+Hyper-V getting to eVMCS levels of enlightenment, my vote is
to put these helpers in svm/nested.c and move then if/when we do end up accumulating
more SVM+Hyper-V code.

As for merge order, I don't think there's a need for this series to take a
dependency on the cleanup, especially if these helpers land in nested.c.  Fixing
up the casting and s/hv_enlightenments/hv_vmcb_enlightenments is straightforward.

[*] https://lore.kernel.org/all/20220921201607.3156750-1-seanjc@google.com