linux-kernel - Re: [PATCH 03/22] kvm: mmu: Init / Uninit the TDP MMU

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Date:   Tue, 29 Sep 2020 22:34:36 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Ben Gardon <bgardon@...gle.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Cannon Matthews <cannonmatthews@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Peter Xu <peterx@...hat.com>, Peter Shier <pshier@...gle.com>,
        Peter Feiner <pfeiner@...gle.com>,
        Junaid Shahid <junaids@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        Yulei Zhang <yulei.kernel@...il.com>,
        Wanpeng Li <kernellwp@...il.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Xiao Guangrong <xiaoguangrong.eric@...il.com>
Subject: Re: [PATCH 03/22] kvm: mmu: Init / Uninit the TDP MMU

Nit on all the shortlogs, can you use "KVM: x86/mmu" instead of "kvm: mmu"?

On Fri, Sep 25, 2020 at 02:22:43PM -0700, Ben Gardon wrote:
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> new file mode 100644
> index 0000000000000..8241e18c111e6
> --- /dev/null
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -0,0 +1,34 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +
> +#include "tdp_mmu.h"
> +
> +static bool __read_mostly tdp_mmu_enabled = true;
> +module_param_named(tdp_mmu, tdp_mmu_enabled, bool, 0644);

Do y'all actually toggle tdp_mmu_enabled while VMs are running?  I can see
having a per-VM capability, or a read-only module param, but a writable
module param is... interesting.

> +static bool is_tdp_mmu_enabled(void)
> +{
> +	if (!READ_ONCE(tdp_mmu_enabled))
> +		return false;
> +
> +	if (WARN_ONCE(!tdp_enabled,
> +		      "Creating a VM with TDP MMU enabled requires TDP."))

This should be enforced, i.e. clear tdp_mmu_enabled if !tdp_enabled.  As is,
it's a user triggerable WARN, which is not good, e.g. with PANIC_ON_WARN.

> +		return false;
> +
> +	return true;
> +}