linux-kernel - Re: [PATCH v4 06/11] x86/vmscape: Move mitigation selection to a switch()

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Message-ID: <c1b67fb1-0ef9-4f23-9e09-c5eecc18f595@suse.com>
Date: Tue, 25 Nov 2025 12:19:32 +0200
From: Nikolay Borisov <nik.borisov@...e.com>
To: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
Cc: x86@...nel.org, David Kaplan <david.kaplan@....com>,
 "H. Peter Anvin" <hpa@...or.com>, Josh Poimboeuf <jpoimboe@...nel.org>,
 Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini
 <pbonzini@...hat.com>, Borislav Petkov <bp@...en8.de>,
 Dave Hansen <dave.hansen@...ux.intel.com>, linux-kernel@...r.kernel.org,
 kvm@...r.kernel.org, Asit Mallick <asit.k.mallick@...el.com>,
 Tao Zhang <tao1.zhang@...el.com>
Subject: Re: [PATCH v4 06/11] x86/vmscape: Move mitigation selection to a
 switch()



On 11/25/25 01:09, Pawan Gupta wrote:
> On Fri, Nov 21, 2025 at 04:27:05PM +0200, Nikolay Borisov wrote:
>>
>>
>> On 11/20/25 08:19, Pawan Gupta wrote:
>>> This ensures that all mitigation modes are explicitly handled, while
>>> keeping the mitigation selection for each mode together. This also prepares
>>> for adding BHB-clearing mitigation mode for VMSCAPE.
>>>
>>> Signed-off-by: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
>>> ---
>>>    arch/x86/kernel/cpu/bugs.c | 22 ++++++++++++++++++----
>>>    1 file changed, 18 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
>>> index 1e9b11198db0fe2483bd17b1327bcfd44a2c1dbf..233594ede19bf971c999f4d3cc0f6f213002c16c 100644
>>> --- a/arch/x86/kernel/cpu/bugs.c
>>> +++ b/arch/x86/kernel/cpu/bugs.c
>>> @@ -3231,17 +3231,31 @@ early_param("vmscape", vmscape_parse_cmdline);
>>>    static void __init vmscape_select_mitigation(void)
>>>    {
>>> -	if (!boot_cpu_has_bug(X86_BUG_VMSCAPE) ||
>>> -	    !boot_cpu_has(X86_FEATURE_IBPB)) {
>>> +	if (!boot_cpu_has_bug(X86_BUG_VMSCAPE)) {
>>>    		vmscape_mitigation = VMSCAPE_MITIGATION_NONE;
>>>    		return;
>>>    	}
>>> -	if (vmscape_mitigation == VMSCAPE_MITIGATION_AUTO) {
>>> -		if (should_mitigate_vuln(X86_BUG_VMSCAPE))
>>> +	if ((vmscape_mitigation == VMSCAPE_MITIGATION_AUTO) &&
>>> +	    !should_mitigate_vuln(X86_BUG_VMSCAPE))
>>> +		vmscape_mitigation = VMSCAPE_MITIGATION_NONE;
>>> +
>>> +	switch (vmscape_mitigation) {
>>> +	case VMSCAPE_MITIGATION_NONE:
>>> +		break;
>>> +
>>> +	case VMSCAPE_MITIGATION_IBPB_ON_VMEXIT:
>>> +	case VMSCAPE_MITIGATION_IBPB_EXIT_TO_USER:
>>> +		if (!boot_cpu_has(X86_FEATURE_IBPB))
>>> +			vmscape_mitigation = VMSCAPE_MITIGATION_NONE;
>>> +		break;
>>> +
>>> +	case VMSCAPE_MITIGATION_AUTO:
>>> +		if (boot_cpu_has(X86_FEATURE_IBPB))
>>>    			vmscape_mitigation = VMSCAPE_MITIGATION_IBPB_EXIT_TO_USER;
>>
>>
>> IMO this patch is a net-negative because as per my reply to patch 9 you have
>> effectively a dead branch:
>>
>> The clear BHB_CLEAR_USER one, however it turns out you have yet another one:
>> VMSCAPE_MITIGATION_IBPB_ON_VMEXIT as it's only ever set in
>> vmscape_update_mitigation() which executes after '_select()' as well and
> 
> Removed VMSCAPE_MITIGATION_IBPB_ON_VMEXIT.
> 
>> additionally you duplicate the FEATURE_IBPB check.
> 
> FEATURE_IBPB check is still needed for VMSCAPE_MITIGATION_IBPB_EXIT_TO_USER.
> I don't think we can drop that.

But if X86_FEATURE_IBPB is not present then all branches boil down to 
setting the mitigation to NONE. What I was suggesting is to not remove 
the that check at the top.

> 
>> So I think either dropping it or removing the superfluous branches is in
>> order.
>>
>>>    		else
>>>    			vmscape_mitigation = VMSCAPE_MITIGATION_NONE;
>>> +		break;
>>>    	}
>>>    }
>>>
>>