KVM: selftests: Adjust VM's initial stack address to align with SysV ABI spec

Align the guest stack to match calling sequence requirements in
section "The Stack Frame" of the System V ABI AMD64 Architecture
Processor Supplement, which requires the value (%rsp + 8), NOT %rsp,
to be a multiple of 16 when control is transferred to the function
entry point. I.e. in a normal function call, %rsp needs to be 16-byte
aligned _before_ CALL, not after.

This fixes unexpected #GPs in guest code when the compiler uses SSE
instructions, e.g. to initialize memory, as many SSE instructions
require memory operands (including those on the stack) to be
16-byte-aligned.

Signed-off-by: Ackerley Tng <ackerleytng@google.com>
Link: https://lore.kernel.org/r/20230227180601.104318-1-ackerleytng@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: Ackerley Tng

tools/testing/selftests/kvm/lib/x86_64/processor.c

@@ -5,6 +5,7 @@
  * Copyright (C) 2018, Google LLC.
  */
 
+#include "linux/bitmap.h"
 #include "test_util.h"
 #include "kvm_util.h"
 #include "processor.h"
@@ -573,14 +574,29 @@ struct kvm_vcpu *vm_arch_vcpu_add(struct kvm_vm *vm, uint32_t vcpu_id,
 				       DEFAULT_GUEST_STACK_VADDR_MIN,
 				       MEM_REGION_DATA);
 
+	stack_vaddr += DEFAULT_STACK_PGS * getpagesize();
+
+	/*
+	 * Align stack to match calling sequence requirements in section "The
+	 * Stack Frame" of the System V ABI AMD64 Architecture Processor
+	 * Supplement, which requires the value (%rsp + 8) to be a multiple of
+	 * 16 when control is transferred to the function entry point.
+	 *
+	 * If this code is ever used to launch a vCPU with 32-bit entry point it
+	 * may need to subtract 4 bytes instead of 8 bytes.
+	 */
+	TEST_ASSERT(IS_ALIGNED(stack_vaddr, PAGE_SIZE),
+		    "__vm_vaddr_alloc() did not provide a page-aligned address");
+	stack_vaddr -= 8;
+
 	vcpu = __vm_vcpu_add(vm, vcpu_id);
 	vcpu_init_cpuid(vcpu, kvm_get_supported_cpuid());
 	vcpu_setup(vm, vcpu);
 
 	/* Setup guest general purpose registers */
 	vcpu_regs_get(vcpu, &regs);
 	regs.rflags = regs.rflags | 0x2;
-	regs.rsp = stack_vaddr + (DEFAULT_STACK_PGS * getpagesize());
+	regs.rsp = stack_vaddr;
 	regs.rip = (unsigned long) guest_code;
 	vcpu_regs_set(vcpu, &regs);