linux-kernel - Allocating SEV C-bit-cleared pages (without relying on swiotlb)

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Message-Id: <b16ec2d7-4a84-482f-b875-d7c152facab5@vates.tech>
Date: Thu, 27 Mar 2025 14:12:28 +0000
From: "Teddy Astie" <teddy.astie@...es.tech>
To: linux-kernel@...r.kernel.org, linux-mm@...r.kernel.org
Cc: Xen-devel <xen-devel@...ts.xenproject.org>
Subject: Allocating SEV C-bit-cleared pages (without relying on swiotlb)

Hello Linux mailing list !

For porting Linux code to make it work on Xen with AMD-SEV, I need to 
change the allocation of some pages to use "shared pages" (C-bit 
cleared) instead of private pages (C-bit set, which is the default kind) 
as these pages needs to be shared with the hypervisor/Dom0.

Is there a facility to allocate pages with C-bit cleared (and if not 
running under SEV, just allocate a plain page) ? Current Linux code for 
SEV seems to only rely on swiotlb as access to shared page is mostly 
made through DMA-kind devices (e.g virtio or emulated device), but I 
don't think it is the best approach.

Regards
Teddy

Teddy Astie | Vates XCP-ng Developer

XCP-ng & Xen Orchestra - Vates solutions

web: https://vates.tech