[PATCH 1/2] x86/fred: Play nice with invoking asm_fred_entry_from_kvm() on non-FRED hardware

[Sean Christopherson]

Modify asm_fred_entry_from_kvm() to allow it to be invoked by KVM even
when FRED isn't fully enabled, e.g. when running with CONFIG_X86_FRED=y
on non-FRED hardware.  This will allow forcing KVM to always use the FRED
entry points for 64-bit kernels, which in turn will eliminate a rather
gross non-CFI indirect call that KVM uses to trampoline IRQs by doing IDT
lookups.

When FRED isn't enabled, simply skip ERETS and restore RBP and RSP from
the stack frame prior to doing a "regular" RET back to KVM (in quotes
because of all the RET mitigation horrors).

Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
 arch/x86/entry/entry_64_fred.S | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/x86/entry/entry_64_fred.S b/arch/x86/entry/entry_64_fred.S
index 29c5c32c16c3..7aff2f0a285f 100644
--- a/arch/x86/entry/entry_64_fred.S
+++ b/arch/x86/entry/entry_64_fred.S
@@ -116,7 +116,8 @@ SYM_FUNC_START(asm_fred_entry_from_kvm)
 	movq %rsp, %rdi				/* %rdi -> pt_regs */
 	call __fred_entry_from_kvm		/* Call the C entry point */
 	POP_REGS
-	ERETS
+
+	ALTERNATIVE "", __stringify(ERETS), X86_FEATURE_FRED
 1:
 	/*
 	 * Objtool doesn't understand what ERETS does, this hint tells it that
@@ -124,7 +125,7 @@ SYM_FUNC_START(asm_fred_entry_from_kvm)
 	 * isn't strictly needed, but it's the simplest form.
 	 */
 	UNWIND_HINT_RESTORE
-	pop %rbp
+	leave
 	RET
 
 SYM_FUNC_END(asm_fred_entry_from_kvm)

base-commit: 45eb29140e68ffe8e93a5471006858a018480a45
-- 
2.49.0.906.g1f30a19c02-goog