linux-kernel - [PATCH v5 2/3] x86/boot/compressed: Skip Video Memory access in Decompressor for SEV-ES/SNP.

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Message-ID: <1398c689a345b3d1751f4c06b5a9356738a296e7.1713222642.git.ashish.kalra@amd.com>
Date: Mon, 15 Apr 2024 23:23:13 +0000
From: Ashish Kalra <Ashish.Kalra@....com>
To: <tglx@...utronix.de>, <mingo@...hat.com>, <bp@...en8.de>,
	<dave.hansen@...ux.intel.com>, <x86@...nel.org>
CC: <rafael@...nel.org>, <peterz@...radead.org>, <adrian.hunter@...el.com>,
	<sathyanarayanan.kuppuswamy@...ux.intel.com>, <jun.nakajima@...el.com>,
	<rick.p.edgecombe@...el.com>, <thomas.lendacky@....com>,
	<michael.roth@....com>, <seanjc@...gle.com>, <kai.huang@...el.com>,
	<bhe@...hat.com>, <kirill.shutemov@...ux.intel.com>, <bdas@...hat.com>,
	<vkuznets@...hat.com>, <dionnaglaze@...gle.com>, <anisinha@...hat.com>,
	<jroedel@...e.de>, <ardb@...nel.org>, <kexec@...ts.infradead.org>,
	<linux-coco@...ts.linux.dev>, <linux-kernel@...r.kernel.org>
Subject: [PATCH v5 2/3] x86/boot/compressed: Skip Video Memory access in Decompressor for SEV-ES/SNP.

From: Ashish Kalra <ashish.kalra@....com>

Accessing guest video memory/RAM during kernel decompressor
causes guest termination as boot stage2 #VC handler for
SEV-ES/SNP systems does not support MMIO handling.

This issue is observed with SEV-ES/SNP guest kexec as
kexec -c adds screen_info to the boot parameters
passed to the kexec kernel, which causes console output to
be dumped to both video and serial.

As the decompressor output gets cleared really fast, it is
preferable to get the console output only on serial, hence,
skip accessing video RAM during decompressor stage to
prevent guest termination.

Serial console output during decompressor stage works as
boot stage2 #VC handler already supports handling port I/O.

Suggested-by: Thomas Lendacy <thomas.lendacky@....com>
Signed-off-by: Ashish Kalra <ashish.kalra@....com>
Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.intel.com>
---
 arch/x86/boot/compressed/misc.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/x86/boot/compressed/misc.c b/arch/x86/boot/compressed/misc.c
index b70e4a21c15f..3b9f96b3dbcc 100644
--- a/arch/x86/boot/compressed/misc.c
+++ b/arch/x86/boot/compressed/misc.c
@@ -427,8 +427,10 @@ asmlinkage __visible void *extract_kernel(void *rmode, unsigned char *output)
 		vidport = 0x3d4;
 	}
 
-	lines = boot_params_ptr->screen_info.orig_video_lines;
-	cols = boot_params_ptr->screen_info.orig_video_cols;
+	if (!(sev_status & MSR_AMD64_SEV_ES_ENABLED)) {
+		lines = boot_params_ptr->screen_info.orig_video_lines;
+		cols = boot_params_ptr->screen_info.orig_video_cols;
+	}
 
 	init_default_io_ops();
 
-- 
2.34.1