linux-kernel - Re: [RFC][PATCH 15/22] x86,vmx: Remove .fixup usage

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Message-ID: <20211106082808.GS174703@worktop.programming.kicks-ass.net>
Date:   Sat, 6 Nov 2021 09:28:08 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, jpoimboe@...hat.com,
        mark.rutland@....com, dvyukov@...gle.com, pbonzini@...hat.com,
        mbenes@...e.cz
Subject: Re: [RFC][PATCH 15/22] x86,vmx: Remove .fixup usage

On Fri, Nov 05, 2021 at 06:17:53PM +0000, Sean Christopherson wrote:

> And hopefully this entire mess will
> go away before that happens, as the trampoline shenanigans are needed
> only because the minimum compiler version doesn't guarantee support for
> asm goto with outputs.

We can at least have those who use sane compilers get sane code..

Something like this, right?

--- a/arch/x86/kvm/vmx/vmx_ops.h
+++ b/arch/x86/kvm/vmx/vmx_ops.h
@@ -70,6 +70,31 @@ static __always_inline unsigned long __v
 {
 	unsigned long value;
 
+#ifdef CONFIG_CC_HAS_ASM_GOTO_OUTPUT
+
+	asm_volatile_goto("1: vmread %[field], %[output]\n\t"
+			  "jna %l[do_fail]\n\t"
+
+			  _ASM_EXTABLE(1b, %l[do_exception])
+
+			  : ASM_CALL_CONSTRAINT, [output] "=r" (value)
+			  : [field] "r" (field)
+			  : "cc"
+			  : do_fail, do_exception);
+
+	return value;
+
+do_fail: __cold;
+	WARN_ONCE(1, "kvm: vmread failed: field=%lx\n", field);
+	pr_warn_ratelimited("kvm: vmread failed: field=%lx\n", field);
+	return 0;
+
+do_exception: __cold;
+	kvm_spurious_fault();
+	return 0;
+
+#else /* !CONFIG_CC_HAS_ASM_GOTO_OUTPUT */
+
 	asm volatile("1: vmread %2, %1\n\t"
 		     ".byte 0x3e\n\t" /* branch taken hint */
 		     "ja 3f\n\t"
@@ -94,7 +119,10 @@ static __always_inline unsigned long __v
 		     _ASM_EXTABLE_TYPE_REG(1b, 2b, EX_TYPE_EFAULT_REG, %1)
 
 		     : ASM_CALL_CONSTRAINT, "=r"(value) : "r"(field) : "cc");
+
 	return value;
+
+#endif /* CONFIG_CC_HAS_ASM_GOTO_OUTPUT */
 }
 
 static __always_inline u16 vmcs_read16(unsigned long field)