Re: [PATCH v9 00/22] Enable FRED with KVM VMX

[David Woodhouse]

On Thu, 2026-05-07 at 15:53 +0200, Maciej Wieczor-Retman wrote:
> 
> My theory is that after 'int3' call the FRED event is handled elsewhere and %rdi
> is not preserved. So the original version of the assembly looks okay but I was
> thinking that int3 has side effects.

Please could you try the version at
https://git.infradead.org/?p=users/dwmw2/linux.git;a=commitdiff;h=fred
or this incremental patch:

diff --git a/tools/testing/selftests/kvm/x86/int1_fred_test.c b/tools/testing/selftests/kvm/x86/int1_fred_test.c
index 7ffb54b56047..b91600abda62 100644
--- a/tools/testing/selftests/kvm/x86/int1_fred_test.c
+++ b/tools/testing/selftests/kvm/x86/int1_fred_test.c
@@ -74,7 +74,7 @@ static volatile uint64_t fred_ss_value;
 static volatile uint64_t fred_saved_rip;
 static volatile bool fred_handler_called;
 
-static void __used fred_handler(struct fred_stack_frame *frame)
+static __attribute__((used)) void fred_handler(struct fred_stack_frame *frame)
 {
 	fred_ss_value = frame->ss;
 	fred_saved_rip = frame->rip;
@@ -99,8 +99,41 @@ asm(
 	/* Pad to +256 for ring 0 entry */
 	".org fred_entrypoint + 256, 0xcc\n"
 "fred_entrypoint_kernel:\n"
-	"movq %rsp, %rdi\n"
+	/* Save all GPRs — exception must be transparent to interrupted code */
+	"pushq %rax\n"
+	"pushq %rcx\n"
+	"pushq %rdx\n"
+	"pushq %rbx\n"
+	"pushq %rbp\n"
+	"pushq %rsi\n"
+	"pushq %rdi\n"
+	"pushq %r8\n"
+	"pushq %r9\n"
+	"pushq %r10\n"
+	"pushq %r11\n"
+	"pushq %r12\n"
+	"pushq %r13\n"
+	"pushq %r14\n"
+	"pushq %r15\n"
+	/* Pass pointer to FRED stack frame (above our saved regs) */
+	"leaq 120(%rsp), %rdi\n"
 	"call fred_handler\n"
+	/* Restore all GPRs */
+	"popq %r15\n"
+	"popq %r14\n"
+	"popq %r13\n"
+	"popq %r12\n"
+	"popq %r11\n"
+	"popq %r10\n"
+	"popq %r9\n"
+	"popq %r8\n"
+	"popq %rdi\n"
+	"popq %rsi\n"
+	"popq %rbp\n"
+	"popq %rbx\n"
+	"popq %rdx\n"
+	"popq %rcx\n"
+	"popq %rax\n"
 	".byte 0xf2, 0x0f, 0x01, 0xca\n"	/* ERETS */
 	".popsection\n"
 );

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