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: Mon, 15 Jan 2024 10:23:18 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Dave Hansen <dave.hansen@...el.com>,
 Tom Lendacky <thomas.lendacky@....com>, Borislav Petkov <bp@...en8.de>
Cc: Michael Roth <michael.roth@....com>, 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, 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>
Subject: Re: [PATCH v1 11/26] x86/sev: Invalidate pages from the direct map
 when adding them to the RMP table

On 1/12/24 21:37, Dave Hansen wrote:
> On 1/12/24 12:28, Tom Lendacky wrote:
>> I thought there was also a desire to remove the direct map for any pages
>> assigned to a guest as private, not just the case that the comment says.
>> So updating the comment would probably the best action.
> 
> I'm not sure who desires that.
> 
> It's sloooooooow to remove things from the direct map.  There's almost
> certainly a frequency cutoff where running the whole direct mapping as
> 4k is better than the cost of mapping/unmapping.
> 
> Actually, where _is_ the TLB flushing here?

Hm yeah it seems to be using the _noflush version? Maybe the RMP issues this
avoids are only triggered with actual page tables and a stray outdated TLB
hit doesn't trigger it? Needs documenting though if that's the case.