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

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Message-ID: <20200330164525.GB21601@ashkalra_ubuntu_server>
Date:   Mon, 30 Mar 2020 16:45:25 +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 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.

> > +		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