From: Ashish Kalra <ashish.kalra@amd.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@amd.com>
Signed-off-by: Ashish Kalra <ashish.kalra@amd.com>
Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.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

On Thu, May 30, 2024 at 11:37:14PM +0000, Ashish Kalra wrote:
> -	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;
> +	}

By now I get an allergic reaction from this sprinkling of "if sev..."
everywhere in the code.

>  	init_default_io_ops();

<--- right here there's a call to

	early_tdx_detect();

You can add a early_sev_detect() counterpart here and clear lines and
cols in it along with an explanation why it is being done.

This is at least a bit cleaner than this.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette