linux-kernel - [PATCH v9 2/3] x86/boot: Skip video memory access in the decompressor for SEV-ES/SNP

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Message-ID: <42f56d2405522cc207b59235506c0b5bec67123f.1718920799.git.ashish.kalra@amd.com>
Date: Thu, 20 Jun 2024 22:23:44 +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>, <hpa@...or.com>, <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>,
	<dyoung@...hat.com>, <kexec@...ts.infradead.org>,
	<linux-coco@...ts.linux.dev>, <linux-kernel@...r.kernel.org>
Subject: [PATCH v9 2/3] x86/boot: Skip video memory access in the decompressor for SEV-ES/SNP

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

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

This issue is observed during a SEV-ES/SNP guest kexec as kexec -c adds
screen_info to the boot parameters passed to the second 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 the 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.

  [ bp: Massage. ]

Suggested-by: Borislav Petkov (AMD) <bp@...en8.de>
Suggested-by: Thomas Lendacy <thomas.lendacky@....com>
Signed-off-by: Ashish Kalra <ashish.kalra@....com>
Signed-off-by: Borislav Petkov (AMD) <bp@...en8.de>
Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.intel.com>
---
 arch/x86/boot/compressed/misc.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/arch/x86/boot/compressed/misc.c b/arch/x86/boot/compressed/misc.c
index 944454306ef4..826b4d5cb1f0 100644
--- a/arch/x86/boot/compressed/misc.c
+++ b/arch/x86/boot/compressed/misc.c
@@ -385,6 +385,19 @@ static void parse_mem_encrypt(struct setup_header *hdr)
 		hdr->xloadflags |= XLF_MEM_ENCRYPTION;
 }
 
+static void early_sev_detect(void)
+{
+	/*
+	 * Accessing video memory causes guest termination because
+	 * the boot stage2 #VC handler of SEV-ES/SNP guests does not
+	 * support MMIO handling and 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.
+	 */
+	if (sev_status & MSR_AMD64_SEV_ES_ENABLED)
+		lines = cols = 0;
+}
+
 /*
  * The compressed kernel image (ZO), has been moved so that its position
  * is against the end of the buffer used to hold the uncompressed kernel
@@ -440,6 +453,8 @@ asmlinkage __visible void *extract_kernel(void *rmode, unsigned char *output)
 	 */
 	early_tdx_detect();
 
+	early_sev_detect();
+
 	console_init();
 
 	/*
-- 
2.34.1