Allow kernel boot on QEMU with AMD SEV.

When booting an OpenBSD kernel on Linux QEMU with SEV enabled, the
hypervisor does not forward the SME feature, only the SEV bit is
set.  Therefore do not depend on SME when checking for SEV guest
mode in locore0.

from Sebastian Sturm; via hshoexer@; OK jsg@

diff --git a/sys/arch/amd64/amd64/locore0.S b/sys/arch/amd64/amd64/locore0.S
index bc45eee..9788217 100644 (file)
--- a/sys/arch/amd64/amd64/locore0.S
+++ b/sys/arch/amd64/amd64/locore0.S
@@ -1,4 +1,4 @@
-/*     $OpenBSD: locore0.S,v 1.25 2024/07/21 19:41:31 bluhm Exp $      */
+/*     $OpenBSD: locore0.S,v 1.26 2024/10/04 21:15:52 bluhm Exp $      */
 /*     $NetBSD: locore.S,v 1.13 2004/03/25 18:33:17 drochner Exp $     */
 
 /*
@@ -269,37 +269,33 @@ cont:
        orl     %edx, RELOC(cpu_feature)
 
        /*
-        * Determine AMD SME and SEV capabilities.
+        * Determine AMD SEV capability.
         */
        movl    $RELOC(cpu_vendor),%ebp
        cmpl $0x68747541, (%ebp)        /* "Auth" */
-       jne     .Lno_smesev
+       jne     .Lno_sev
        cmpl $0x69746e65, 4(%ebp)       /* "enti" */
-       jne     .Lno_smesev
+       jne     .Lno_sev
        cmpl $0x444d4163, 8(%ebp)       /* "cAMD" */
-       jne     .Lno_smesev
+       jne     .Lno_sev
 
-       /* AMD CPU, check for SME and SEV. */
+       /* AMD CPU, check for SEV. */
        movl    $0x8000001f, %eax
        cpuid
-       pushl   %eax
-       andl    $CPUIDEAX_SME, %eax     /* SME */
-       popl    %eax
-       jz      .Lno_smesev
        andl    $CPUIDEAX_SEV, %eax     /* SEV */
-       jz      .Lno_smesev
+       jz      .Lno_sev
 
        /* Are we in guest mode with SEV enabled? */
        movl    $MSR_SEV_STATUS, %ecx
        rdmsr
        andl    $SEV_STAT_ENABLED, %eax
-       jz      .Lno_smesev
+       jz      .Lno_sev
 
        /* Determine C bit position */
        movl    %ebx, %ecx      /* %ebx from previous cpuid */
        andl    $0x3f, %ecx
        cmpl    $0x20, %ecx     /* must be at least bit 32 (counting from 0) */
-       jl      .Lno_smesev
+       jl      .Lno_sev
        xorl    %eax, %eax
        movl    %eax, RELOC(pg_crypt)
        subl    $0x20, %ecx
@@ -338,7 +334,7 @@ cont:
 
        movl    $0x1, RELOC(cpu_sev_guestmode)  /* we are a SEV guest */
 
-.Lno_smesev:
+.Lno_sev:
 
        /*
         * Finished with old stack; load new %esp now instead of later so we