linux-kernel - Re: [RFC Part2 PATCH 02/30] x86/sev-snp: add RMP entry lookup helpers

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Message-ID: <20210415195020.GG6318@zn.tnic>
Date:   Thu, 15 Apr 2021 21:50:20 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Brijesh Singh <brijesh.singh@....com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org, kvm@...r.kernel.org,
        linux-crypto@...r.kernel.org, ak@...ux.intel.com,
        herbert@...dor.apana.org.au, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Joerg Roedel <jroedel@...e.de>,
        "H. Peter Anvin" <hpa@...or.com>, Tony Luck <tony.luck@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        David Rientjes <rientjes@...gle.com>,
        Sean Christopherson <seanjc@...gle.com>
Subject: Re: [RFC Part2 PATCH 02/30] x86/sev-snp: add RMP entry lookup helpers

On Thu, Apr 15, 2021 at 01:08:09PM -0500, Brijesh Singh wrote:
> This is from Family 19h Model 01h Rev B01. The processor which
> introduces the SNP feature. Yes, I have already upload the PPR on the BZ.
> 
> The PPR is also available at AMD: https://www.amd.com/en/support/tech-docs

Please add the link in the bugzilla to the comments here - this is the
reason why stuff is being uploaded in the first place, because those
vendor sites tend to change and those links become stale with time.

> I guess I was trying to shorten the name. I am good with struct rmpentry;

Yes please - typedefs are used only in very specific cases.

> All those magic numbers are documented in the PPR.

We use defines - not magic numbers. For example

#define RMPTABLE_ENTRIES_OFFSET 0x4000

The 8 is probably

PAGE_SHIFT - RMPENTRY_SHIFT

because you have GPA bits [50:12] and an RMP entry is 16 bytes, i.e., 1 << 4.

With defines it is actually clear what the computation is doing - with
naked numbers not really.

> APM does not provide the offset of the entry inside the RMP table.

It does, kinda, but in the pseudocode of those new insns in APM v3. From
PVALIDATE pseudo:

	RMP_ENTRY_PA = RMP_BASE + 0x4000 + (SYSTEM_PA / 0x1000) * 16

and that last

	/ 0x1000 * 16

is actually

	>> 12 - 4

i.e., the >> 8 shift.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette