linux-kernel - Re: [PATCH] x86/kvm: Override default caching mode for SEV-SNP and TDX

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Message-ID: <2263013c-4cfe-4ed8-ab24-f01f15bb41ca@suse.com>
Date: Tue, 15 Oct 2024 14:37:18 +0200
From: Jürgen Groß <jgross@...e.com>
To: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>,
 Vitaly Kuznetsov <vkuznets@...hat.com>, Thomas Gleixner
 <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>, kvm@...r.kernel.org,
 linux-kernel@...r.kernel.org, Binbin Wu <binbin.wu@...el.com>,
 Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [PATCH] x86/kvm: Override default caching mode for SEV-SNP and
 TDX

On 15.10.24 14:31, Kirill A. Shutemov wrote:
> On Tue, Oct 15, 2024 at 12:12:51PM +0200, Jürgen Groß wrote:
>> On 15.10.24 11:58, Kirill A. Shutemov wrote:
>>> AMD SEV-SNP and Intel TDX have limited access to MTRR: either it is not
>>> advertised in CPUID or it cannot be programmed (on TDX, due to #VE on
>>> CR0.CD clear).
>>>
>>> This results in guests using uncached mappings where it shouldn't and
>>> pmd/pud_set_huge() failures due to non-uniform memory type reported by
>>> mtrr_type_lookup().
>>>
>>> Override MTRR state, making it WB by default as the kernel does for
>>> Hyper-V guests.
>>>
>>> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
>>> Suggested-by: Binbin Wu <binbin.wu@...el.com>
>>> Cc: Juergen Gross <jgross@...e.com>
>>> Cc: Tom Lendacky <thomas.lendacky@....com>
>>> ---
>>>    arch/x86/kernel/kvm.c | 4 ++++
>>>    1 file changed, 4 insertions(+)
>>>
>>> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
>>> index 263f8aed4e2c..21e9e4845354 100644
>>> --- a/arch/x86/kernel/kvm.c
>>> +++ b/arch/x86/kernel/kvm.c
>>> @@ -37,6 +37,7 @@
>>>    #include <asm/apic.h>
>>>    #include <asm/apicdef.h>
>>>    #include <asm/hypervisor.h>
>>> +#include <asm/mtrr.h>
>>>    #include <asm/tlb.h>
>>>    #include <asm/cpuidle_haltpoll.h>
>>>    #include <asm/ptrace.h>
>>> @@ -980,6 +981,9 @@ static void __init kvm_init_platform(void)
>>>    	}
>>>    	kvmclock_init();
>>>    	x86_platform.apic_post_init = kvm_apic_init;
>>> +
>>> +	/* Set WB as the default cache mode for SEV-SNP and TDX */
>>> +	mtrr_overwrite_state(NULL, 0, MTRR_TYPE_WRBACK);
>>
>> Do you really want to do this for _all_ KVM guests?
>>
>> I'd expect this call to be conditional on TDX or SEV-SNP.
> 
> mtrr_overwrite_state() checks it internally.

Ah, right, I forgot I added that check on request by Boris. :-)

Reviewed-by: Juergen Gross <jgross@...e.com>


Juergen


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