linux-kernel - Re: [PATCH -v1.2] kvm/emulate: Fix SETcc emulation function offsets with SLS

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Date:   Sun, 20 Mar 2022 15:04:02 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Jamie Heilman <jamie@...ible.transient.net>,
        linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        Sean Christopherson <seanjc@...gle.com>, kvm@...r.kernel.org
Subject: Re: [PATCH -v1.2] kvm/emulate: Fix SETcc emulation function offsets
 with SLS

On 3/19/22 14:50, Borislav Petkov wrote:
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index 5719d8cfdbd9..f321abb9a4a8 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -429,8 +429,11 @@ static int fastop(struct x86_emulate_ctxt *ctxt, fastop_t fop);
>   	FOP_END
>   
>   /* Special case for SETcc - 1 instruction per cc */
> +
> +#define SETCC_ALIGN	(4 * (1 + IS_ENABLED(CONFIG_SLS)))
> +
>   #define FOP_SETCC(op) \
> -	".align 4 \n\t" \
> +	".align " __stringify(SETCC_ALIGN) " \n\t" \
>   	".type " #op ", @function \n\t" \
>   	#op ": \n\t" \
>   	#op " %al \n\t" \
> @@ -1047,7 +1050,7 @@ static int em_bsr_c(struct x86_emulate_ctxt *ctxt)
>   static __always_inline u8 test_cc(unsigned int condition, unsigned long flags)
>   {
>   	u8 rc;
> -	void (*fop)(void) = (void *)em_setcc + 4 * (condition & 0xf);
> +	void (*fop)(void) = (void *)em_setcc + SETCC_ALIGN * (condition & 0xf);
>   
>   	flags = (flags & EFLAGS_MASK) | X86_EFLAGS_IF;
>   	asm("push %[flags]; popf; " CALL_NOSPEC

So this is what I squashed in:

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index f321abb9a4a8..e86d610dc6b7 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -430,7 +430,19 @@ static int fastop(struct x86_emulate_ctxt *ctxt, fastop_t fop);
  
  /* Special case for SETcc - 1 instruction per cc */
  
-#define SETCC_ALIGN	(4 * (1 + IS_ENABLED(CONFIG_SLS)))
+/*
+ * Depending on .config the SETcc functions look like:
+ *
+ * SETcc %al   [3 bytes]
+ * RET         [1 byte]
+ * INT3        [1 byte; CONFIG_SLS]
+ *
+ * Which gives possible sizes 4 or 5.  When rounded up to the
+ * next power-of-two alignment they become 4 or 8.
+ */
+#define SETCC_LENGTH	(4 + IS_ENABLED(CONFIG_SLS))
+#define SETCC_ALIGN	(4 << IS_ENABLED(CONFIG_SLS))
+static_assert(SETCC_LENGTH <= SETCC_ALIGN);
  
  #define FOP_SETCC(op) \
  	".align " __stringify(SETCC_ALIGN) " \n\t" \

Paolo