linux-kernel - Re: [PATCH v2 21/50] KVM: MIPS: Hardcode callbacks to hardware virtualization extensions

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Date:   Thu, 1 Dec 2022 23:00:13 +0100
From:   Philippe Mathieu-Daudé <philmd@...aro.org>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Marc Zyngier <maz@...nel.org>,
        Huacai Chen <chenhuacai@...nel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@...il.com>,
        Anup Patel <anup@...infault.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Matthew Rosato <mjrosato@...ux.ibm.com>,
        Eric Farman <farman@...ux.ibm.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        David Woodhouse <dwmw2@...radead.org>,
        Paul Durrant <paul@....org>
Cc:     James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Oliver Upton <oliver.upton@...ux.dev>,
        Atish Patra <atishp@...shpatra.org>,
        David Hildenbrand <david@...hat.com>, kvm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
        kvmarm@...ts.cs.columbia.edu, linux-mips@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, kvm-riscv@...ts.infradead.org,
        linux-riscv@...ts.infradead.org, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org, Yuan Yao <yuan.yao@...el.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Isaku Yamahata <isaku.yamahata@...el.com>,
        Fabiano Rosas <farosas@...ux.ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Kai Huang <kai.huang@...el.com>, Chao Gao <chao.gao@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2 21/50] KVM: MIPS: Hardcode callbacks to hardware
 virtualization extensions

On 1/12/22 00:09, Sean Christopherson wrote:
> Now that KVM no longer supports trap-and-emulate (see commit 45c7e8af4a5e
> "MIPS: Remove KVM_TE support"), hardcode the MIPS callbacks to the
> virtualization callbacks.
> 
> Harcoding the callbacks eliminates the technically-unnecessary check on
> non-NULL kvm_mips_callbacks in kvm_arch_init().  MIPS has never supported
> multiple in-tree modules, i.e. barring an out-of-tree module, where
> copying and renaming kvm.ko counts as "out-of-tree", KVM could never
> encounter a non-NULL set of callbacks during module init.
> 
> The callback check is also subtly broken, as it is not thread safe,
> i.e. if there were multiple modules, loading both concurrently would
> create a race between checking and setting kvm_mips_callbacks.
> 
> Given that out-of-tree shenanigans are not the kernel's responsibility,
> hardcode the callbacks to simplify the code.
> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>   arch/mips/include/asm/kvm_host.h |  2 +-
>   arch/mips/kvm/Makefile           |  2 +-
>   arch/mips/kvm/callback.c         | 14 --------------
>   arch/mips/kvm/mips.c             |  9 ++-------
>   arch/mips/kvm/vz.c               |  7 ++++---
>   5 files changed, 8 insertions(+), 26 deletions(-)
>   delete mode 100644 arch/mips/kvm/callback.c
> 
> diff --git a/arch/mips/include/asm/kvm_host.h b/arch/mips/include/asm/kvm_host.h
> index 28f0ba97db71..2803c9c21ef9 100644
> --- a/arch/mips/include/asm/kvm_host.h
> +++ b/arch/mips/include/asm/kvm_host.h
> @@ -758,7 +758,7 @@ struct kvm_mips_callbacks {
>   	void (*vcpu_reenter)(struct kvm_vcpu *vcpu);
>   };
>   extern struct kvm_mips_callbacks *kvm_mips_callbacks;

IIUC we could even constify this pointer.

Anyway,
Reviewed-by: Philippe Mathieu-Daudé <philmd@...aro.org>

> diff --git a/arch/mips/kvm/mips.c b/arch/mips/kvm/mips.c
> index af29490d9740..f0a6c245d1ff 100644
> --- a/arch/mips/kvm/mips.c
> +++ b/arch/mips/kvm/mips.c
> @@ -1012,17 +1012,12 @@ long kvm_arch_vm_ioctl(struct file *filp, unsigned int ioctl, unsigned long arg)
>   
>   int kvm_arch_init(void *opaque)
>   {
> -	if (kvm_mips_callbacks) {
> -		kvm_err("kvm: module already exists\n");
> -		return -EEXIST;
> -	}
> -
> -	return kvm_mips_emulation_init(&kvm_mips_callbacks);
> +	return kvm_mips_emulation_init();
>   }
>   
>   void kvm_arch_exit(void)
>   {
> -	kvm_mips_callbacks = NULL;
> +
>   }
>   
>   int kvm_arch_vcpu_ioctl_get_sregs(struct kvm_vcpu *vcpu,
> diff --git a/arch/mips/kvm/vz.c b/arch/mips/kvm/vz.c
> index c706f5890a05..dafab003ea0d 100644
> --- a/arch/mips/kvm/vz.c
> +++ b/arch/mips/kvm/vz.c
> @@ -3304,7 +3304,10 @@ static struct kvm_mips_callbacks kvm_vz_callbacks = {
>   	.vcpu_reenter = kvm_vz_vcpu_reenter,
>   };
>   
> -int kvm_mips_emulation_init(struct kvm_mips_callbacks **install_callbacks)
> +/* FIXME: Get rid of the callbacks now that trap-and-emulate is gone. */
> +struct kvm_mips_callbacks *kvm_mips_callbacks = &kvm_vz_callbacks;