Date:   Mon, 13 Feb 2023 19:43:14 +0100
From:   Borislav Petkov <bp@alien8.de>
To:     Juergen Gross <jgross@suse.com>
Cc:     linux-kernel@vger.kernel.org, x86@kernel.org,
        lists@nerdbynature.de, mikelley@microsoft.com,
        torvalds@linux-foundation.org,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH v2 2/8] x86/mtrr: support setting MTRR state for software
 defined MTRRs
Message-ID: <Y+qEwtzCV65s+ZFq@zn.tnic>
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On Mon, Feb 13, 2023 at 04:36:12PM +0100, Juergen Gross wrote:
> In the end I wouldn't mind dropping the fixed MTRRs from the interface, as
> they are currently not needed at all.

Yes, the less the better.

> I'd say we go with what is needed right now. And having a single interface
> makes all the sanity checking you are asking for easier.

I guess I need to remember to finish designing this if more users
appear...

> What are you especially asking for?
> 
> With my current patches Xen PV dom0 will call mtrr_overwrite_state() before
> x86_hyper_type is set, while a Hyper-V SEV-SNP guest will make the call after
> it has been set. Both calls happen before cache_bp_init().
> 
> So I could move the mtrr_overwrite_state() call for Xen PV dom0 into its
> init_platform() callback and check in mtrr_overwrite_state() x86_hyper_type
> to be set,

I believe that is good enough, see below.

> or I could reject a call of mtrr_overwrite_state() after the call of
> cache_bp_init() has happened, or I could do both.

I think one thing is enough as we'll be loud enough.

---
diff --git a/arch/x86/kernel/cpu/mtrr/mtrr.c b/arch/x86/kernel/cpu/mtrr/mtrr.c
index b73fe243c7fd..2dbe2c10e959 100644
--- a/arch/x86/kernel/cpu/mtrr/mtrr.c
+++ b/arch/x86/kernel/cpu/mtrr/mtrr.c
@@ -49,6 +49,7 @@
 #include <asm/cacheinfo.h>
 #include <asm/cpufeature.h>
 #include <asm/e820/api.h>
+#include <asm/hypervisor.h>
 #include <asm/mtrr.h>
 #include <asm/msr.h>
 #include <asm/memtype.h>
@@ -668,7 +669,12 @@ void __init mtrr_bp_init(void)
 	const char *why = "(not available)";
 	unsigned int phys_addr;
 
+#ifdef CONFIG_HYPERVISOR_GUEST
 	if (mtrr_state.enabled) {
+
+		/* This should not happen without a hypervisor present. */
+		WARN_ON_ONCE(!x86_hyper_type);
+
 		/* Software overwrite of MTRR state, only for generic case. */
 		mtrr_calc_physbits(true);
 		init_table();
@@ -676,6 +682,7 @@ void __init mtrr_bp_init(void)
 
 		return;
 	}
+#endif
 
 	phys_addr = mtrr_calc_physbits(boot_cpu_has(X86_FEATURE_MTRR));
 

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette