linux-kernel - Re: [PATCH] KVM: x86: compile out TDP MMU on 32-bit systems

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Date:   Mon, 8 Feb 2021 11:26:13 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: compile out TDP MMU on 32-bit systems

On Sat, Feb 06, 2021, Paolo Bonzini wrote:
> The TDP MMU assumes that it can do atomic accesses to 64-bit PTEs.
> Rather than just disabling it, compile it out completely so that it
> is possible to use for example 64-bit xchg.
> 
> To limit the number of stubs, wrap all accesses to tdp_mmu_enabled
> or tdp_mmu_page with a function.  Calls to all other functions in
> tdp_mmu.c are eliminated and do not even reach the linker.

Aha!  I always forget how smart the compiler can (sometimes) be.  I agree this
isn't at all painful.

This can/should also expand the #ifdef to the TDP-only fields in kvm_mmu_page.
I also vote to #ifdef out all of tdp_iter.h, and probably the TDP-only fields in
struct kvm_arch.

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index e0171f7176c5..84499aad01a4 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1033,6 +1033,7 @@ struct kvm_arch {
        struct kvm_pmu_event_filter *pmu_event_filter;
        struct task_struct *nx_lpage_recovery_thread;

+#ifdef CONFIG_X86_64
        /*
         * Whether the TDP MMU is enabled for this VM. This contains a
         * snapshot of the TDP MMU module parameter from when the VM was
@@ -1071,6 +1072,7 @@ struct kvm_arch {
         * the thread holds the MMU lock in write mode.
         */
        spinlock_t tdp_mmu_pages_lock;
+#endif /* CONFIG_X86_64 */
 };

 struct kvm_vm_stat {
diff --git a/arch/x86/kvm/mmu/mmu_internal.h b/arch/x86/kvm/mmu/mmu_internal.h
index 98db78a26957..9e38d3c5daad 100644
--- a/arch/x86/kvm/mmu/mmu_internal.h
+++ b/arch/x86/kvm/mmu/mmu_internal.h
@@ -56,10 +56,12 @@ struct kvm_mmu_page {
        /* Number of writes since the last time traversal visited this page.  */
        atomic_t write_flooding_count;

+#ifdef CONFIG_X86_64
        bool tdp_mmu_page;

        /* Used for freeing the page asyncronously if it is a TDP MMU page. */
        struct rcu_head rcu_head;
+#endif
 };

 extern struct kmem_cache *mmu_page_header_cache;
diff --git a/arch/x86/kvm/mmu/tdp_iter.h b/arch/x86/kvm/mmu/tdp_iter.h
index 4cc177d75c4a..5f60c1b1a1b4 100644
--- a/arch/x86/kvm/mmu/tdp_iter.h
+++ b/arch/x86/kvm/mmu/tdp_iter.h
@@ -7,6 +7,8 @@

 #include "mmu.h"

+#ifdef CONFIG_X86_64
+
 typedef u64 __rcu *tdp_ptep_t;

 /*
@@ -64,4 +66,6 @@ void tdp_iter_start(struct tdp_iter *iter, u64 *root_pt, int root_level,
 void tdp_iter_next(struct tdp_iter *iter);
 tdp_ptep_t tdp_iter_root_pt(struct tdp_iter *iter);

+#endif /* CONFIG_X86_64 */
+
 #endif /* __KVM_X86_MMU_TDP_ITER_H */



With the above:

Reviewed-and-tested-by: Sean Christopherson <seanjc@...gle.com>

> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>