[kvm-unit-tests,RFC,13/16] x86 AMD SEV-ES: Load GDT with UEFI segments

Commit Message

Zixuan Wang Aug. 18, 2021, 12:09 a.m. UTC

Before this commit, KVM-Unit-Tests set up process crashes when executing
'lgdt' instruction under SEV-ES. This is because lgdt triggers UEFI
procedures (e.g. UEFI #VC handler) that require UEFI's code and data
segments. But these segments are are not compatible with KVM-Unit-Tests
GDT:

UEFI uses 0x30 as code segment and 0x38 as data segment, but in
KVM-Unit-Tests' GDT, 0x30 is a data segment, and 0x38 is a code segment.
This discrepancy crashes the UEFI procedures and thus crashes the 'lgdt'
execution.

This commit fixes this issue by copying UEFI GDT's code and data
segments into KVM-Unit-Tests GDT, so that UEFI procedures (e.g. UEFI #VC
handler) can work.

In this commit, the guest VM passes setup_gdt_tss() but crashes in
load_idt(), which will be fixed by follow-up commits.

Signed-off-by: Zixuan Wang <[e-mail]>
---
 lib/x86/amd_sev.c | 36 ++++++++++++++++++++++++++++++++++++
 lib/x86/amd_sev.h |  1 +
 lib/x86/setup.c   |  4 ++++
 3 files changed, 41 insertions(+)

Patch

diff --git a/lib/x86/amd_sev.c b/lib/x86/amd_sev.c
index a31d352..c2aebdf 100644
--- a/lib/x86/amd_sev.c
+++ b/lib/x86/amd_sev.c
@@ -53,6 +53,42 @@  EFI_STATUS setup_amd_sev_es(void){
 
 	return EFI_SUCCESS;
 }
+
+static void copy_gdt_entry(gdt_entry_t *dst, gdt_entry_t *src, unsigned segment)
+{
+	unsigned index;
+
+	index = segment / sizeof(gdt_entry_t);
+	dst[index] = src[index];
+}
+
+/* Defined in x86/efi/efistart64.S */
+extern gdt_entry_t gdt64[];
+
+/* Copy UEFI's code and data segments to KVM-Unit-Tests GDT.
+ *
+ * This is because KVM-Unit-Tests reuses UEFI #VC handler that requires UEFI
+ * code and data segments to run. The UEFI #VC handler crashes the guest VM if
+ * these segments are not available. So we need to copy these two UEFI segments
+ * into KVM-Unit-Tests GDT.
+ *
+ * UEFI uses 0x30 as code segment and 0x38 as data segment. Fortunately, these
+ * segments can be safely overridden in KVM-Unit-Tests as they are used as
+ * protected mode and real mode segments (see x86/efi/efistart64.S for more
+ * details), which are not used in EFI set up process.
+ */
+void copy_uefi_segments(void)
+{
+	/* GDT and GDTR in current UEFI */
+	gdt_entry_t *gdt_curr;
+	struct descriptor_table_ptr gdtr_curr;
+
+	/* Copy code and data segments from UEFI */
+	sgdt(&gdtr_curr);
+	gdt_curr = (gdt_entry_t *)gdtr_curr.base;
+	copy_gdt_entry(gdt64, gdt_curr, read_cs());
+	copy_gdt_entry(gdt64, gdt_curr, read_ds());
+}
 #endif /* CONFIG_AMD_SEV_ES */
 
 unsigned long long get_amd_sev_c_bit_mask(void)
diff --git a/lib/x86/amd_sev.h b/lib/x86/amd_sev.h
index a2eccfc..4d81cae 100644
--- a/lib/x86/amd_sev.h
+++ b/lib/x86/amd_sev.h
@@ -39,6 +39,7 @@ 
 EFI_STATUS setup_amd_sev(void);
 #ifdef CONFIG_AMD_SEV_ES
 EFI_STATUS setup_amd_sev_es(void);
+void copy_uefi_segments(void);
 #endif /* CONFIG_AMD_SEV_ES */
 
 unsigned long long get_amd_sev_c_bit_mask(void);
diff --git a/lib/x86/setup.c b/lib/x86/setup.c
index d29f415..d828638 100644
--- a/lib/x86/setup.c
+++ b/lib/x86/setup.c
@@ -348,6 +348,10 @@  static void setup_gdt_tss(void)
 	tss_hi->limit_low = (u16)((curr_tss_addr >> 32) & 0xffff);
 	tss_hi->base_low = (u16)((curr_tss_addr >> 48) & 0xffff);
 
+#ifdef CONFIG_AMD_SEV_ES
+	copy_uefi_segments();
+#endif /* CONFIG_AMD_SEV_ES */
+
 	load_gdt_tss(tss_offset);
 }