linux-kernel - Re: [PATCH v1 11/26] x86/sev: Invalidate pages from the direct map when adding them to the RMP table

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Date: Tue, 16 Jan 2024 12:22:39 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: Michael Roth <michael.roth@....com>
Cc: Tom Lendacky <thomas.lendacky@....com>, Borislav Petkov <bp@...en8.de>,
 x86@...nel.org, kvm@...r.kernel.org, linux-coco@...ts.linux.dev,
 linux-mm@...ck.org, linux-crypto@...r.kernel.org,
 linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com,
 jroedel@...e.de, hpa@...or.com, ardb@...nel.org, pbonzini@...hat.com,
 seanjc@...gle.com, vkuznets@...hat.com, jmattson@...gle.com,
 luto@...nel.org, dave.hansen@...ux.intel.com, slp@...hat.com,
 pgonda@...gle.com, peterz@...radead.org,
 srinivas.pandruvada@...ux.intel.com, rientjes@...gle.com, tobin@....com,
 vbabka@...e.cz, kirill@...temov.name, ak@...ux.intel.com,
 tony.luck@...el.com, sathyanarayanan.kuppuswamy@...ux.intel.com,
 alpergun@...gle.com, jarkko@...nel.org, ashish.kalra@....com,
 nikunj.dadhania@....com, pankaj.gupta@....com, liam.merwick@...cle.com,
 zhi.a.wang@...el.com, Brijesh Singh <brijesh.singh@....com>, rppt@...nel.org
Subject: Re: [PATCH v1 11/26] x86/sev: Invalidate pages from the direct map
 when adding them to the RMP table

On 1/16/24 08:19, Michael Roth wrote:
> 
> So at the very least, if we went down this path, we would be worth
> investigating the following areas in addition to general perf testing:
> 
>   1) Only splitting directmap regions corresponding to kernel-allocatable
>      *data* (hopefully that's even feasible...)

Take a look at the 64-bit memory map in here:

	https://www.kernel.org/doc/Documentation/x86/x86_64/mm.rst

We already have separate mappings for kernel data and (normal) kernel text.

>   2) Potentially deferring the split until an SNP guest is actually
>      run, so there isn't any impact just from having SNP enabled (though
>      you still take a hit from RMP checks in that case so maybe it's not
>      worthwhile, but that itself has been noted as a concern for users
>      so it would be nice to not make things even worse).

Yes, this would be nice too.

>> Actually, where _is_ the TLB flushing here?
> Boris pointed that out in v6, and we implemented it in v7, but it
> completely cratered performance:

That *desperately* needs to be documented.

How can it be safe to skip the TLB flush?  It this akin to a page
permission promotion where you go from RO->RW but can skip the TLB
flush?  In that case, the CPU will see the RO TLB entry violation, drop
it, and re-walk the page tables, discovering the RW entry.

Does something similar happen here where the CPU sees the 2M/4k conflict
in the TLB, drops the 2M entry, does a re-walk then picks up the
newly-split 2M->4k entries?

I can see how something like that would work, but it's _awfully_ subtle
to go unmentioned.