linux-kernel - Re: [PATCH v6 14/14] KVM: x86: Add kexec support for SEV Live Migration.

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Date:   Tue, 31 Mar 2020 09:26:26 -0500
From:   Brijesh Singh <brijesh.singh@....com>
To:     Ashish Kalra <ashish.kalra@....com>
Cc:     brijesh.singh@....com, pbonzini@...hat.com, tglx@...utronix.de,
        mingo@...hat.com, hpa@...or.com, joro@...tes.org, bp@...e.de,
        thomas.lendacky@....com, x86@...nel.org, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, rientjes@...gle.com,
        srutherford@...gle.com, luto@...nel.org
Subject: Re: [PATCH v6 14/14] KVM: x86: Add kexec support for SEV Live
 Migration.


On 3/30/20 11:45 AM, Ashish Kalra wrote:
> Hello Brijesh,
>
> On Mon, Mar 30, 2020 at 11:00:14AM -0500, Brijesh Singh wrote:
>> On 3/30/20 1:23 AM, Ashish Kalra wrote:
>>> From: Ashish Kalra <ashish.kalra@....com>
>>>
>>> Reset the host's page encryption bitmap related to kernel
>>> specific page encryption status settings before we load a
>>> new kernel by kexec. We cannot reset the complete
>>> page encryption bitmap here as we need to retain the
>>> UEFI/OVMF firmware specific settings.
>>>
>>> Signed-off-by: Ashish Kalra <ashish.kalra@....com>
>>> ---
>>>  arch/x86/kernel/kvm.c | 28 ++++++++++++++++++++++++++++
>>>  1 file changed, 28 insertions(+)
>>>
>>> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
>>> index 8fcee0b45231..ba6cce3c84af 100644
>>> --- a/arch/x86/kernel/kvm.c
>>> +++ b/arch/x86/kernel/kvm.c
>>> @@ -34,6 +34,7 @@
>>>  #include <asm/hypervisor.h>
>>>  #include <asm/tlb.h>
>>>  #include <asm/cpuidle_haltpoll.h>
>>> +#include <asm/e820/api.h>
>>>  
>>>  static int kvmapf = 1;
>>>  
>>> @@ -357,6 +358,33 @@ static void kvm_pv_guest_cpu_reboot(void *unused)
>>>  	 */
>>>  	if (kvm_para_has_feature(KVM_FEATURE_PV_EOI))
>>>  		wrmsrl(MSR_KVM_PV_EOI_EN, 0);
>>> +	/*
>>> +	 * Reset the host's page encryption bitmap related to kernel
>>> +	 * specific page encryption status settings before we load a
>>> +	 * new kernel by kexec. NOTE: We cannot reset the complete
>>> +	 * page encryption bitmap here as we need to retain the
>>> +	 * UEFI/OVMF firmware specific settings.
>>> +	 */
>>> +	if (kvm_para_has_feature(KVM_FEATURE_SEV_LIVE_MIGRATION) &&
>>> +		(smp_processor_id() == 0)) {
>>
>> In patch 13/14, the KVM_FEATURE_SEV_LIVE_MIGRATION is set
>> unconditionally and because of that now the below code will be executed
>> on non-SEV guest. IMO, this feature must be cleared for non-SEV guest to
>> avoid making unnecessary hypercall's.
>>
>>
> I will additionally add a sev_active() check here to ensure that we don't make the unnecassary hypercalls on non-SEV guests.


IMO, instead of using the sev_active() we should make sure that the
feature is not enabled when SEV is not active.


>>> +		unsigned long nr_pages;
>>> +		int i;
>>> +
>>> +		for (i = 0; i < e820_table->nr_entries; i++) {
>>> +			struct e820_entry *entry = &e820_table->entries[i];
>>> +			unsigned long start_pfn, end_pfn;
>>> +
>>> +			if (entry->type != E820_TYPE_RAM)
>>> +				continue;
>>> +
>>> +			start_pfn = entry->addr >> PAGE_SHIFT;
>>> +			end_pfn = (entry->addr + entry->size) >> PAGE_SHIFT;
>>> +			nr_pages = DIV_ROUND_UP(entry->size, PAGE_SIZE);
>>> +
>>> +			kvm_sev_hypercall3(KVM_HC_PAGE_ENC_STATUS,
>>> +				entry->addr, nr_pages, 1);
>>> +		}
>>> +	}
>>>  	kvm_pv_disable_apf();
>>>  	kvm_disable_steal_time();
>>>  }
> Thanks,
> Ashish