linux-kernel - Re: [PATCH v10 7/9] KVM: VMX: Implement and wire get_untagged

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Date:   Thu, 17 Aug 2023 17:51:27 +0800
From:   Binbin Wu <binbin.wu@...ux.intel.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        pbonzini@...hat.com, chao.gao@...el.com, kai.huang@...el.com,
        David.Laight@...lab.com, robert.hu@...ux.intel.com,
        guang.zeng@...el.com
Subject: Re: [PATCH v10 7/9] KVM: VMX: Implement and wire get_untagged_addr()
 for LAM



On 8/17/2023 6:01 AM, Sean Christopherson wrote:
> On Wed, Jul 19, 2023, Binbin Wu wrote:
>> +	return (sign_extend64(gva, lam_bit) & ~BIT_ULL(63)) | (gva & BIT_ULL(63));
> Almost forgot.  Please add a comment explaning how LAM untags the address,
> specifically the whole bit 63 preservation.  The logic is actually straightforward,
> but the above looks way more complex than it actually is.  This?
>
> 	/*
> 	 * Untag the address by sign-extending the LAM bit, but NOT to bit 63.
> 	 * Bit 63 is retained from the raw virtual address so that untagging
> 	 * doesn't change a user access to a supervisor access, and vice versa.
> 	 */
OK.

Besides it, I find I forgot adding the comments for the function. I will 
add it back if you don't object.

+/*
+ * Only called in 64-bit mode.
+ *
+ * LAM has a modified canonical check when applicable:
+ * LAM_S48                : [ 1 ][ metadata ][ 1 ]
+ *                            63               47
+ * LAM_U48                : [ 0 ][ metadata ][ 0 ]
+ *                            63               47
+ * LAM_S57                : [ 1 ][ metadata ][ 1 ]
+ *                            63               56
+ * LAM_U57 + 5-lvl paging : [ 0 ][ metadata ][ 0 ]
+ *                            63               56
+ * LAM_U57 + 4-lvl paging : [ 0 ][ metadata ][ 0...0 ]
+ *                            63               56..47
+ *
+ * Note that KVM masks the metadata in addresses, performs the (original)
+ * canonicality checking and then walks page table. This is slightly
+ * different from hardware behavior but achieves the same effect.
+ * Specifically, if LAM is enabled, the processor performs a modified
+ * canonicality checking where the metadata are ignored instead of
+ * masked. After the modified canonicality checking, the processor masks
+ * the metadata before passing addresses for paging translation.
+ */