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>

diff --git a/tools/testing/selftests/kvm/lib/x86_64/processor.c b/tools/testing/selftests/kvm/lib/x86_64/processor.c
index c39a4353ba194c13257e1aaae62c0ee6889c776d..c09cd151fbe099aadbfd13687b2e834b9c7e2f6d 100644 (file)
--- a/tools/testing/selftests/kvm/lib/x86_64/processor.c
+++ b/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"
@@ -573,6 +574,21 @@ 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);
@@ -580,7 +596,7 @@ struct kvm_vcpu *vm_arch_vcpu_add(struct kvm_vm *vm, uint32_t vcpu_id,
        /* 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);