linux-kernel - Re: [PATCH 2/4] KVM: x86/mmu: Defer "full" MMU setup until after vendor hardware

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Date:   Sat, 25 Jun 2022 00:16:05 +0000
From:   David Matlack <dmatlack@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Peter Xu <peterx@...hat.com>
Subject: Re: [PATCH 2/4] KVM: x86/mmu: Defer "full" MMU setup until after
 vendor hardware_setup()

On Fri, Jun 24, 2022 at 11:27:33PM +0000, Sean Christopherson wrote:
> Defer MMU setup, and in particular allocation of pte_list_desc_cache,
> until after the vendor's hardware_setup() has run, i.e. until after the
> MMU has been configured by vendor code.  This will allow a future commit
> to dynamically size pte_list_desc's array of sptes based on whether or
> not KVM is using TDP.
> 
> Alternatively, the setup could be done in kvm_configure_mmu(), but that
> would require vendor code to call e.g. kvm_unconfigure_mmu() in teardown
> and error paths, i.e. doesn't actually save code and is arguably uglier.
> 
> Note, keep the reset of PTE masks where it is to ensure that the masks
> are reset before the vendor's hardware_setup() runs, i.e. before the
> vendor code has a chance to manipulate the masks, e.g. VMX modifies masks
> even before calling kvm_configure_mmu().
> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
[...]
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 17ac30b9e22c..ceb81e04aea3 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -6673,10 +6673,8 @@ void kvm_mmu_x86_module_init(void)
>   * loaded as many of the masks/values may be modified by VMX or SVM, i.e. need
>   * to be reset when a potentially different vendor module is loaded.
>   */
> -int kvm_mmu_vendor_module_init(void)
> +void kvm_mmu_vendor_module_init(void)
>  {
> -	int ret = -ENOMEM;
> -
>  	/*
>  	 * MMU roles use union aliasing which is, generally speaking, an
>  	 * undefined behavior. However, we supposedly know how compilers behave
> @@ -6687,7 +6685,13 @@ int kvm_mmu_vendor_module_init(void)
>  	BUILD_BUG_ON(sizeof(union kvm_mmu_extended_role) != sizeof(u32));
>  	BUILD_BUG_ON(sizeof(union kvm_cpu_role) != sizeof(u64));
>  
> +	/* Reset the PTE masks before the vendor module's hardware setup. */
>  	kvm_mmu_reset_all_pte_masks();
> +}
> +
> +int kvm_mmu_hardware_setup(void)
> +{

Instead of putting this code in a new function and calling it after
hardware_setup(), we could put it in kvm_configure_mmu().

This will result in a larger patch diff, but has it eliminates a subtle
and non-trivial-to-verify dependency ordering between
kvm_configure_mmu() and kvm_mmu_hardware_setup() and it will co-locate
the initialization of nr_sptes_per_pte_list and the code that uses it to
create pte_list_desc_cache in a single function.