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Peter Anvin" , Joerg Roedel , Borislav Petkov , Tom Lendacky , X86 ML , KVM list , LKML , David Rientjes , Andy Lutomirski Subject: Re: [PATCH v6 08/14] KVM: X86: Introduce KVM_HC_PAGE_ENC_STATUS hypercall Message-ID: <20200409161812.GA18882@ashkalra_ubuntu_server> References: <265ef8a0ab75f01bc673cce6ddcf7988c7623943.1585548051.git.ashish.kalra@amd.com> <20200407052740.GA31821@ashkalra_ubuntu_server> <20200408011726.GA3684@ashkalra_ubuntu_server> <42597534-b8c6-4c73-9b12-ddbde079fc7c@amd.com> <20200408031818.GA27654@ashkalra_ubuntu_server> Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20200408031818.GA27654@ashkalra_ubuntu_server> User-Agent: Mutt/1.9.4 (2018-02-28) X-ClientProxiedBy: DM5PR21CA0054.namprd21.prod.outlook.com (2603:10b6:3:129::16) To DM5PR12MB1386.namprd12.prod.outlook.com (2603:10b6:3:77::9) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from ashkalra_ubuntu_server (165.204.77.1) by DM5PR21CA0054.namprd21.prod.outlook.com (2603:10b6:3:129::16) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.2921.6 via Frontend Transport; Thu, 9 Apr 2020 16:18:16 +0000 X-Originating-IP: [165.204.77.1] X-MS-PublicTrafficType: Email X-MS-Office365-Filtering-HT: Tenant X-MS-Office365-Filtering-Correlation-Id: 50909678-b8d3-4013-345d-08d7dca19cea X-MS-TrafficTypeDiagnostic: DM5PR12MB1660:|DM5PR12MB1660: X-MS-Exchange-Transport-Forked: True X-Microsoft-Antispam-PRVS: X-MS-Oob-TLC-OOBClassifiers: OLM:6430; X-Forefront-PRVS: 0368E78B5B X-Forefront-Antispam-Report: CIP:255.255.255.255;CTRY:;LANG:en;SCL:1;SRV:;IPV:NLI;SFV:NSPM;H:DM5PR12MB1386.namprd12.prod.outlook.com;PTR:;CAT:NONE;SFTY:;SFS:(10009020)(4636009)(136003)(376002)(39860400002)(396003)(366004)(346002)(55016002)(186003)(33716001)(26005)(66556008)(316002)(66476007)(478600001)(16526019)(66946007)(81166007)(6862004)(6666004)(5660300002)(44832011)(86362001)(66574012)(53546011)(6636002)(52116002)(1076003)(956004)(8936002)(966005)(4326008)(33656002)(8676002)(2906002)(9686003)(81156014)(54906003)(6496006)(30864003)(7416002);DIR:OUT;SFP:1101; Received-SPF: None (protection.outlook.com: amd.com does not designate permitted sender hosts) X-MS-Exchange-SenderADCheck: 1 X-Microsoft-Antispam: BCL:0; X-Microsoft-Antispam-Message-Info: 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 X-MS-Exchange-AntiSpam-MessageData: fJYF/U9ljLGPzvLKlp5P60sSzfxM9zk4ofxscNOpQ5zHwLZvXxxSXaQQe3iPqLWtCk0vjVj7lXL+J1G9Bk+tvwdSWdSPm7A+gyyK16A68VvR+WF6gpQnZwol0gIbhmBoOj7upLz/E7iq0Ad3U/BneA== X-OriginatorOrg: amd.com X-MS-Exchange-CrossTenant-Network-Message-Id: 50909678-b8d3-4013-345d-08d7dca19cea X-MS-Exchange-CrossTenant-OriginalArrivalTime: 09 Apr 2020 16:18:18.1042 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: 3dd8961f-e488-4e60-8e11-a82d994e183d X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: y8shW5tNGLoGQLdH81GfLqaX2AO3aDPvnK4riBeJsK9ZbolvSFh2D9z146Yavt4p+3ckZrpJZZH6+sbqyECKWg== X-MS-Exchange-Transport-CrossTenantHeadersStamped: DM5PR12MB1660 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hello Brijesh, Steve, On Wed, Apr 08, 2020 at 03:18:18AM +0000, Ashish Kalra wrote: > Hello Brijesh, > > On Tue, Apr 07, 2020 at 09:34:15PM -0500, Brijesh Singh wrote: > > > > On 4/7/20 8:38 PM, Steve Rutherford wrote: > > > On Tue, Apr 7, 2020 at 6:17 PM Ashish Kalra wrote: > > >> Hello Steve, Brijesh, > > >> > > >> On Tue, Apr 07, 2020 at 05:35:57PM -0700, Steve Rutherford wrote: > > >>> On Tue, Apr 7, 2020 at 5:29 PM Brijesh Singh wrote: > > >>>> > > >>>> On 4/7/20 7:01 PM, Steve Rutherford wrote: > > >>>>> On Mon, Apr 6, 2020 at 10:27 PM Ashish Kalra wrote: > > >>>>>> Hello Steve, > > >>>>>> > > >>>>>> On Mon, Apr 06, 2020 at 07:17:37PM -0700, Steve Rutherford wrote: > > >>>>>>> On Sun, Mar 29, 2020 at 11:22 PM Ashish Kalra wrote: > > >>>>>>>> From: Brijesh Singh > > >>>>>>>> > > >>>>>>>> This hypercall is used by the SEV guest to notify a change in the page > > >>>>>>>> encryption status to the hypervisor. The hypercall should be invoked > > >>>>>>>> only when the encryption attribute is changed from encrypted -> decrypted > > >>>>>>>> and vice versa. By default all guest pages are considered encrypted. > > >>>>>>>> > > >>>>>>>> Cc: Thomas Gleixner > > >>>>>>>> Cc: Ingo Molnar > > >>>>>>>> Cc: "H. Peter Anvin" > > >>>>>>>> Cc: Paolo Bonzini > > >>>>>>>> Cc: "Radim Krčmář" > > >>>>>>>> Cc: Joerg Roedel > > >>>>>>>> Cc: Borislav Petkov > > >>>>>>>> Cc: Tom Lendacky > > >>>>>>>> Cc: x86@kernel.org > > >>>>>>>> Cc: kvm@vger.kernel.org > > >>>>>>>> Cc: linux-kernel@vger.kernel.org > > >>>>>>>> Signed-off-by: Brijesh Singh > > >>>>>>>> Signed-off-by: Ashish Kalra > > >>>>>>>> --- > > >>>>>>>> Documentation/virt/kvm/hypercalls.rst | 15 +++++ > > >>>>>>>> arch/x86/include/asm/kvm_host.h | 2 + > > >>>>>>>> arch/x86/kvm/svm.c | 95 +++++++++++++++++++++++++++ > > >>>>>>>> arch/x86/kvm/vmx/vmx.c | 1 + > > >>>>>>>> arch/x86/kvm/x86.c | 6 ++ > > >>>>>>>> include/uapi/linux/kvm_para.h | 1 + > > >>>>>>>> 6 files changed, 120 insertions(+) > > >>>>>>>> > > >>>>>>>> diff --git a/Documentation/virt/kvm/hypercalls.rst b/Documentation/virt/kvm/hypercalls.rst > > >>>>>>>> index dbaf207e560d..ff5287e68e81 100644 > > >>>>>>>> --- a/Documentation/virt/kvm/hypercalls.rst > > >>>>>>>> +++ b/Documentation/virt/kvm/hypercalls.rst > > >>>>>>>> @@ -169,3 +169,18 @@ a0: destination APIC ID > > >>>>>>>> > > >>>>>>>> :Usage example: When sending a call-function IPI-many to vCPUs, yield if > > >>>>>>>> any of the IPI target vCPUs was preempted. > > >>>>>>>> + > > >>>>>>>> + > > >>>>>>>> +8. KVM_HC_PAGE_ENC_STATUS > > >>>>>>>> +------------------------- > > >>>>>>>> +:Architecture: x86 > > >>>>>>>> +:Status: active > > >>>>>>>> +:Purpose: Notify the encryption status changes in guest page table (SEV guest) > > >>>>>>>> + > > >>>>>>>> +a0: the guest physical address of the start page > > >>>>>>>> +a1: the number of pages > > >>>>>>>> +a2: encryption attribute > > >>>>>>>> + > > >>>>>>>> + Where: > > >>>>>>>> + * 1: Encryption attribute is set > > >>>>>>>> + * 0: Encryption attribute is cleared > > >>>>>>>> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h > > >>>>>>>> index 98959e8cd448..90718fa3db47 100644 > > >>>>>>>> --- a/arch/x86/include/asm/kvm_host.h > > >>>>>>>> +++ b/arch/x86/include/asm/kvm_host.h > > >>>>>>>> @@ -1267,6 +1267,8 @@ struct kvm_x86_ops { > > >>>>>>>> > > >>>>>>>> bool (*apic_init_signal_blocked)(struct kvm_vcpu *vcpu); > > >>>>>>>> int (*enable_direct_tlbflush)(struct kvm_vcpu *vcpu); > > >>>>>>>> + int (*page_enc_status_hc)(struct kvm *kvm, unsigned long gpa, > > >>>>>>>> + unsigned long sz, unsigned long mode); > > >>>>>>> Nit: spell out size instead of sz. > > >>>>>>>> }; > > >>>>>>>> > > >>>>>>>> struct kvm_arch_async_pf { > > >>>>>>>> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c > > >>>>>>>> index 7c2721e18b06..1d8beaf1bceb 100644 > > >>>>>>>> --- a/arch/x86/kvm/svm.c > > >>>>>>>> +++ b/arch/x86/kvm/svm.c > > >>>>>>>> @@ -136,6 +136,8 @@ struct kvm_sev_info { > > >>>>>>>> int fd; /* SEV device fd */ > > >>>>>>>> unsigned long pages_locked; /* Number of pages locked */ > > >>>>>>>> struct list_head regions_list; /* List of registered regions */ > > >>>>>>>> + unsigned long *page_enc_bmap; > > >>>>>>>> + unsigned long page_enc_bmap_size; > > >>>>>>>> }; > > >>>>>>>> > > >>>>>>>> struct kvm_svm { > > >>>>>>>> @@ -1991,6 +1993,9 @@ static void sev_vm_destroy(struct kvm *kvm) > > >>>>>>>> > > >>>>>>>> sev_unbind_asid(kvm, sev->handle); > > >>>>>>>> sev_asid_free(sev->asid); > > >>>>>>>> + > > >>>>>>>> + kvfree(sev->page_enc_bmap); > > >>>>>>>> + sev->page_enc_bmap = NULL; > > >>>>>>>> } > > >>>>>>>> > > >>>>>>>> static void avic_vm_destroy(struct kvm *kvm) > > >>>>>>>> @@ -7593,6 +7598,94 @@ static int sev_receive_finish(struct kvm *kvm, struct kvm_sev_cmd *argp) > > >>>>>>>> return ret; > > >>>>>>>> } > > >>>>>>>> > > >>>>>>>> +static int sev_resize_page_enc_bitmap(struct kvm *kvm, unsigned long new_size) > > >>>>>>>> +{ > > >>>>>>>> + struct kvm_sev_info *sev = &to_kvm_svm(kvm)->sev_info; > > >>>>>>>> + unsigned long *map; > > >>>>>>>> + unsigned long sz; > > >>>>>>>> + > > >>>>>>>> + if (sev->page_enc_bmap_size >= new_size) > > >>>>>>>> + return 0; > > >>>>>>>> + > > >>>>>>>> + sz = ALIGN(new_size, BITS_PER_LONG) / 8; > > >>>>>>>> + > > >>>>>>>> + map = vmalloc(sz); > > >>>>>>>> + if (!map) { > > >>>>>>>> + pr_err_once("Failed to allocate encrypted bitmap size %lx\n", > > >>>>>>>> + sz); > > >>>>>>>> + return -ENOMEM; > > >>>>>>>> + } > > >>>>>>>> + > > >>>>>>>> + /* mark the page encrypted (by default) */ > > >>>>>>>> + memset(map, 0xff, sz); > > >>>>>>>> + > > >>>>>>>> + bitmap_copy(map, sev->page_enc_bmap, sev->page_enc_bmap_size); > > >>>>>>>> + kvfree(sev->page_enc_bmap); > > >>>>>>>> + > > >>>>>>>> + sev->page_enc_bmap = map; > > >>>>>>>> + sev->page_enc_bmap_size = new_size; > > >>>>>>>> + > > >>>>>>>> + return 0; > > >>>>>>>> +} > > >>>>>>>> + > > >>>>>>>> +static int svm_page_enc_status_hc(struct kvm *kvm, unsigned long gpa, > > >>>>>>>> + unsigned long npages, unsigned long enc) > > >>>>>>>> +{ > > >>>>>>>> + struct kvm_sev_info *sev = &to_kvm_svm(kvm)->sev_info; > > >>>>>>>> + kvm_pfn_t pfn_start, pfn_end; > > >>>>>>>> + gfn_t gfn_start, gfn_end; > > >>>>>>>> + int ret; > > >>>>>>>> + > > >>>>>>>> + if (!sev_guest(kvm)) > > >>>>>>>> + return -EINVAL; > > >>>>>>>> + > > >>>>>>>> + if (!npages) > > >>>>>>>> + return 0; > > >>>>>>>> + > > >>>>>>>> + gfn_start = gpa_to_gfn(gpa); > > >>>>>>>> + gfn_end = gfn_start + npages; > > >>>>>>>> + > > >>>>>>>> + /* out of bound access error check */ > > >>>>>>>> + if (gfn_end <= gfn_start) > > >>>>>>>> + return -EINVAL; > > >>>>>>>> + > > >>>>>>>> + /* lets make sure that gpa exist in our memslot */ > > >>>>>>>> + pfn_start = gfn_to_pfn(kvm, gfn_start); > > >>>>>>>> + pfn_end = gfn_to_pfn(kvm, gfn_end); > > >>>>>>>> + > > >>>>>>>> + if (is_error_noslot_pfn(pfn_start) && !is_noslot_pfn(pfn_start)) { > > >>>>>>>> + /* > > >>>>>>>> + * Allow guest MMIO range(s) to be added > > >>>>>>>> + * to the page encryption bitmap. > > >>>>>>>> + */ > > >>>>>>>> + return -EINVAL; > > >>>>>>>> + } > > >>>>>>>> + > > >>>>>>>> + if (is_error_noslot_pfn(pfn_end) && !is_noslot_pfn(pfn_end)) { > > >>>>>>>> + /* > > >>>>>>>> + * Allow guest MMIO range(s) to be added > > >>>>>>>> + * to the page encryption bitmap. > > >>>>>>>> + */ > > >>>>>>>> + return -EINVAL; > > >>>>>>>> + } > > >>>>>>>> + > > >>>>>>>> + mutex_lock(&kvm->lock); > > >>>>>>>> + ret = sev_resize_page_enc_bitmap(kvm, gfn_end); > > >>>>>>>> + if (ret) > > >>>>>>>> + goto unlock; > > >>>>>>>> + > > >>>>>>>> + if (enc) > > >>>>>>>> + __bitmap_set(sev->page_enc_bmap, gfn_start, > > >>>>>>>> + gfn_end - gfn_start); > > >>>>>>>> + else > > >>>>>>>> + __bitmap_clear(sev->page_enc_bmap, gfn_start, > > >>>>>>>> + gfn_end - gfn_start); > > >>>>>>>> + > > >>>>>>>> +unlock: > > >>>>>>>> + mutex_unlock(&kvm->lock); > > >>>>>>>> + return ret; > > >>>>>>>> +} > > >>>>>>>> + > > >>>>>>>> static int svm_mem_enc_op(struct kvm *kvm, void __user *argp) > > >>>>>>>> { > > >>>>>>>> struct kvm_sev_cmd sev_cmd; > > >>>>>>>> @@ -7995,6 +8088,8 @@ static struct kvm_x86_ops svm_x86_ops __ro_after_init = { > > >>>>>>>> .need_emulation_on_page_fault = svm_need_emulation_on_page_fault, > > >>>>>>>> > > >>>>>>>> .apic_init_signal_blocked = svm_apic_init_signal_blocked, > > >>>>>>>> + > > >>>>>>>> + .page_enc_status_hc = svm_page_enc_status_hc, > > >>>>>>>> }; > > >>>>>>>> > > >>>>>>>> static int __init svm_init(void) > > >>>>>>>> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c > > >>>>>>>> index 079d9fbf278e..f68e76ee7f9c 100644 > > >>>>>>>> --- a/arch/x86/kvm/vmx/vmx.c > > >>>>>>>> +++ b/arch/x86/kvm/vmx/vmx.c > > >>>>>>>> @@ -8001,6 +8001,7 @@ static struct kvm_x86_ops vmx_x86_ops __ro_after_init = { > > >>>>>>>> .nested_get_evmcs_version = NULL, > > >>>>>>>> .need_emulation_on_page_fault = vmx_need_emulation_on_page_fault, > > >>>>>>>> .apic_init_signal_blocked = vmx_apic_init_signal_blocked, > > >>>>>>>> + .page_enc_status_hc = NULL, > > >>>>>>>> }; > > >>>>>>>> > > >>>>>>>> static void vmx_cleanup_l1d_flush(void) > > >>>>>>>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > > >>>>>>>> index cf95c36cb4f4..68428eef2dde 100644 > > >>>>>>>> --- a/arch/x86/kvm/x86.c > > >>>>>>>> +++ b/arch/x86/kvm/x86.c > > >>>>>>>> @@ -7564,6 +7564,12 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu) > > >>>>>>>> kvm_sched_yield(vcpu->kvm, a0); > > >>>>>>>> ret = 0; > > >>>>>>>> break; > > >>>>>>>> + case KVM_HC_PAGE_ENC_STATUS: > > >>>>>>>> + ret = -KVM_ENOSYS; > > >>>>>>>> + if (kvm_x86_ops->page_enc_status_hc) > > >>>>>>>> + ret = kvm_x86_ops->page_enc_status_hc(vcpu->kvm, > > >>>>>>>> + a0, a1, a2); > > >>>>>>>> + break; > > >>>>>>>> default: > > >>>>>>>> ret = -KVM_ENOSYS; > > >>>>>>>> break; > > >>>>>>>> diff --git a/include/uapi/linux/kvm_para.h b/include/uapi/linux/kvm_para.h > > >>>>>>>> index 8b86609849b9..847b83b75dc8 100644 > > >>>>>>>> --- a/include/uapi/linux/kvm_para.h > > >>>>>>>> +++ b/include/uapi/linux/kvm_para.h > > >>>>>>>> @@ -29,6 +29,7 @@ > > >>>>>>>> #define KVM_HC_CLOCK_PAIRING 9 > > >>>>>>>> #define KVM_HC_SEND_IPI 10 > > >>>>>>>> #define KVM_HC_SCHED_YIELD 11 > > >>>>>>>> +#define KVM_HC_PAGE_ENC_STATUS 12 > > >>>>>>>> > > >>>>>>>> /* > > >>>>>>>> * hypercalls use architecture specific > > >>>>>>>> -- > > >>>>>>>> 2.17.1 > > >>>>>>>> > > >>>>>>> I'm still not excited by the dynamic resizing. I believe the guest > > >>>>>>> hypercall can be called in atomic contexts, which makes me > > >>>>>>> particularly unexcited to see a potentially large vmalloc on the host > > >>>>>>> followed by filling the buffer. Particularly when the buffer might be > > >>>>>>> non-trivial in size (~1MB per 32GB, per some back of the envelope > > >>>>>>> math). > > >>>>>>> > > >>>>>> I think looking at more practical situations, most hypercalls will > > >>>>>> happen during the boot stage, when device specific initializations are > > >>>>>> happening, so typically the maximum page encryption bitmap size would > > >>>>>> be allocated early enough. > > >>>>>> > > >>>>>> In fact, initial hypercalls made by OVMF will probably allocate the > > >>>>>> maximum page bitmap size even before the kernel comes up, especially > > >>>>>> as they will be setting up page enc/dec status for MMIO, ROM, ACPI > > >>>>>> regions, PCI device memory, etc., and most importantly for > > >>>>>> "non-existent" high memory range (which will probably be the > > >>>>>> maximum size page encryption bitmap allocated/resized). > > >>>>>> > > >>>>>> Let me know if you have different thoughts on this ? > > >>>>> Hi Ashish, > > >>>>> > > >>>>> If this is not an issue in practice, we can just move past this. If we > > >>>>> are basically guaranteed that OVMF will trigger hypercalls that expand > > >>>>> the bitmap beyond the top of memory, then, yes, that should work. That > > >>>>> leaves me slightly nervous that OVMF might regress since it's not > > >>>>> obvious that calling a hypercall beyond the top of memory would be > > >>>>> "required" for avoiding a somewhat indirectly related issue in guest > > >>>>> kernels. > > >>>> > > >>>> If possible then we should try to avoid growing/shrinking the bitmap . > > >>>> Today OVMF may not be accessing beyond memory but a malicious guest > > >>>> could send a hypercall down which can trigger a huge memory allocation > > >>>> on the host side and may eventually cause denial of service for other. > > >>> Nice catch! Was just writing up an email about this. > > >>>> I am in favor if we can find some solution to handle this case. How > > >>>> about Steve's suggestion about VMM making a call down to the kernel to > > >>>> tell how big the bitmap should be? Initially it should be equal to the > > >>>> guest RAM and if VMM ever did the memory expansion then it can send down > > >>>> another notification to increase the bitmap ? > > >>>> > > >>>> Optionally, instead of adding a new ioctl, I was wondering if we can > > >>>> extend the kvm_arch_prepare_memory_region() to make svm specific x86_ops > > >>>> which can take read the userspace provided memory region and calculate > > >>>> the amount of guest RAM managed by the KVM and grow/shrink the bitmap > > >>>> based on that information. I have not looked deep enough to see if its > > >>>> doable but if it can work then we can avoid adding yet another ioctl. > > >>> We also have the set bitmap ioctl in a later patch in this series. We > > >>> could also use the set ioctl for initialization (it's a little > > >>> excessive for initialization since there will be an additional > > >>> ephemeral allocation and a few additional buffer copies, but that's > > >>> probably fine). An enable_cap has the added benefit of probably being > > >>> necessary anyway so usermode can disable the migration feature flag. > > >>> > > >>> In general, userspace is going to have to be in direct control of the > > >>> buffer and its size. > > >> My only practical concern about setting a static bitmap size based on guest > > >> memory is about the hypercalls being made initially by OVMF to set page > > >> enc/dec status for ROM, ACPI regions and especially the non-existent > > >> high memory range. The new ioctl will statically setup bitmap size to > > >> whatever guest RAM is specified, say 4G, 8G, etc., but the OVMF > > >> hypercall for non-existent memory will try to do a hypercall for guest > > >> physical memory range like ~6G->64G (for 4G guest RAM setup), this > > >> hypercall will basically have to just return doing nothing, because > > >> the allocated bitmap won't have this guest physical range available ? > > > > > > IMO, Ovmf issuing a hypercall beyond the guest RAM is simple wrong, it > > should *not* do that.  There was a feature request I submitted sometime > > back to Tianocore https://bugzilla.tianocore.org/show_bug.cgi?id=623 as > > I saw this coming in future. I tried highlighting the problem in the > > MdeModulePkg that it does not provide a notifier to tell OVMF when core > > creates the MMIO holes etc. It was not a big problem with the SEV > > initially because we were never getting down to hypervisor to do > > something about those non-existent regions. But with the migration its > > now important that we should restart the discussion with UEFI folks and > > see what can be done. In the kernel patches we should do what is right > > for the kernel and not workaround the Ovmf limitation. > > Ok, this makes sense. I will start exploring > kvm_arch_prepare_memory_region() to see if it can assist in computing > the guest RAM or otherwise i will look at adding a new ioctl interface > for the same. > I looked at kvm_arch_prepare_memory_region() and kvm_arch_commit_memory_region() and kvm_arch_commit_memory_region() looks to be ideal to use for this. I walked the kvm_memslots in this function and i can compute the approximate guest RAM mapped by KVM, though, i get the guest RAM size as "twice" the configured size because of the two address spaces on x86 KVM, i believe there is one additional address space for SMM/SMRAM use. I don't think we have a use case of migrating a SEV guest with SMM support ? Considering that, i believe that i can just compute the guest RAM size using memslots for address space #0 and use that to grow/shrink the bitmap. As you mentioned i will need to add a new SVM specific x86_ops to callback as part of kvm_arch_commit_memory_region() which will in-turn call sev_resize_page_enc_bitmap(). Thanks, Ashish > > > > > > >> Also, hypercalls for ROM, ACPI, device regions and any memory holes within > > >> the static bitmap setup as per guest RAM config will work, but what > > >> about hypercalls for any device regions beyond the guest RAM config ? > > >> > > >> Thanks, > > >> Ashish > > > I'm not super familiar with what the address beyond the top of ram is > > > used for. If the memory is not backed by RAM, will it even matter for > > > migration? Sounds like the encryption for SEV won't even apply to it. > > > If we don't need to know what the c-bit state of an address is, we > > > don't need to track it. It doesn't hurt to track it (which is why I'm > > > not super concerned about tracking the memory holes).