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

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Date:   Thu, 2 Apr 2020 23:34:20 +0000
From:   Ashish Kalra <ashish.kalra@....com>
To:     Brijesh Singh <brijesh.singh@....com>
Cc:     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.

Hello Brijesh,

On Tue, Mar 31, 2020 at 09:26:26AM -0500, Brijesh Singh wrote:
> 
> 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.
> 

Yes, now the KVM_FEATURE_SEV_LIVE_MIGRATION feature is enabled
dynamically in svm_cpuid_update() after it gets called from
svm_launch_finish(), which ensures that it only gets set when a SEV
guest is active.

Thanks,
Ashish

> 
> >>> +		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