[PATCH] kvm: x86: Adjust the location of pkru_mask of kvm_mmu to reduce memory

[PATCH] kvm: x86: Adjust the location of pkru_mask of kvm_mmu to reduce memory

[Peng Hao]

From: Peng Hao <flyingpeng@tencent.com>

Adjust the field pkru_mask to the back of direct_map to make up 8-byte
alignment.This reduces the size of kvm_mmu by 8 bytes.

Signed-off-by: Peng Hao <flyingpeng@tencent.com>
---
 arch/x86/include/asm/kvm_host.h | 17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index e5d8700319cc..73dcb09abd5d 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -433,14 +433,6 @@ struct kvm_mmu {
 	u8 shadow_root_level;
 	u8 ept_ad;
 	bool direct_map;
-	struct kvm_mmu_root_info prev_roots[KVM_MMU_NUM_PREV_ROOTS];
-
-	/*
-	 * Bitmap; bit set = permission fault
-	 * Byte index: page fault error code [4:1]
-	 * Bit index: pte permissions in ACC_* format
-	 */
-	u8 permissions[16];
 
 	/*
 	* The pkru_mask indicates if protection key checks are needed.  It
@@ -450,6 +442,15 @@ struct kvm_mmu {
 	*/
 	u32 pkru_mask;
 
+	struct kvm_mmu_root_info prev_roots[KVM_MMU_NUM_PREV_ROOTS];
+
+	/*
+	 * Bitmap; bit set = permission fault
+	 * Byte index: page fault error code [4:1]
+	 * Bit index: pte permissions in ACC_* format
+	 */
+	u8 permissions[16];
+
 	u64 *pae_root;
 	u64 *pml4_root;
 	u64 *pml5_root;
-- 
2.27.0

Re: [PATCH] kvm: x86: Adjust the location of pkru_mask of kvm_mmu to reduce memory

[Sean Christopherson]

On Mon, Feb 28, 2022, Peng Hao wrote:
> From: Peng Hao <flyingpeng@tencent.com>
> 
> Adjust the field pkru_mask to the back of direct_map to make up 8-byte
> alignment.This reduces the size of kvm_mmu by 8 bytes.

I'd prefer we just burn the extra 8 bytes, at least for now.  'prku' and 'permissions'
are related fields, IMO splitting them to save 24 bytes per vCPU isn't a good tradeoff.

And in the not-too-distant future, all of the 1-byte fields will hopefully go
away, at which point were "wasting" 4 bytes no matter where 'pkru' is defined.  'ept_ad'
can be dropped immediately, I'll send a patch for that.  We should be able to drop
'root_level', 'shadow_root_level', and 'direct_map' once Paolo's clean up of the role
stuff lands.

Re: [PATCH] kvm: x86: Adjust the location of pkru_mask of kvm_mmu to reduce memory

[Paolo Bonzini]

Queued, thanks.

Paolo