Date:   Thu, 11 Aug 2022 22:48:28 +0530
From:   "Nikunj A. Dadhania" <nikunj@....com>
To:     "Gupta, Pankaj" <pankaj.gupta@....com>,
        Chao Peng <chao.p.peng@...ux.intel.com>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>,
        Hugh Dickins <hughd@...gle.com>,
        Jeff Layton <jlayton@...nel.org>,
        "J . Bruce Fields" <bfields@...ldses.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Shuah Khan <shuah@...nel.org>, Mike Rapoport <rppt@...nel.org>,
        Steven Price <steven.price@....com>,
        "Maciej S . Szmigiero" <mail@...iej.szmigiero.name>,
        Vlastimil Babka <vbabka@...e.cz>,
        Vishal Annapurve <vannapurve@...gle.com>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        luto@...nel.org, jun.nakajima@...el.com, dave.hansen@...el.com,
        ak@...ux.intel.com, david@...hat.com, aarcange@...hat.com,
        ddutile@...hat.com, dhildenb@...hat.com,
        Quentin Perret <qperret@...gle.com>,
        Michael Roth <michael.roth@....com>, mhocko@...e.com,
        Muchun Song <songmuchun@...edance.com>, bharata@....com,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-kselftest@...r.kernel.org,
        linux-api@...r.kernel.org, linux-doc@...r.kernel.org,
        qemu-devel@...gnu.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v7 00/14] KVM: mm: fd-based approach for supporting KVM
 guest private memory

On 11/08/22 17:00, Gupta, Pankaj wrote:
> 
>>> This is the v7 of this series which tries to implement the fd-based KVM
>>> guest private memory. The patches are based on latest kvm/queue branch
>>> commit:
>>>
>>>    b9b71f43683a (kvm/queue) KVM: x86/mmu: Buffer nested MMU
>>> split_desc_cache only by default capacity
>>>
>>> Introduction
>>> ------------
>>> In general this patch series introduce fd-based memslot which provides
>>> guest memory through memory file descriptor fd[offset,size] instead of
>>> hva/size. The fd can be created from a supported memory filesystem
>>> like tmpfs/hugetlbfs etc. which we refer as memory backing store. KVM
>>> and the the memory backing store exchange callbacks when such memslot
>>> gets created. At runtime KVM will call into callbacks provided by the
>>> backing store to get the pfn with the fd+offset. Memory backing store
>>> will also call into KVM callbacks when userspace punch hole on the fd
>>> to notify KVM to unmap secondary MMU page table entries.
>>>
>>> Comparing to existing hva-based memslot, this new type of memslot allows
>>> guest memory unmapped from host userspace like QEMU and even the kernel
>>> itself, therefore reduce attack surface and prevent bugs.
>>>
>>> Based on this fd-based memslot, we can build guest private memory that
>>> is going to be used in confidential computing environments such as Intel
>>> TDX and AMD SEV. When supported, the memory backing store can provide
>>> more enforcement on the fd and KVM can use a single memslot to hold both
>>> the private and shared part of the guest memory.
>>>
>>> mm extension
>>> ---------------------
>>> Introduces new MFD_INACCESSIBLE flag for memfd_create(), the file
>>> created with these flags cannot read(), write() or mmap() etc via normal
>>> MMU operations. The file content can only be used with the newly
>>> introduced memfile_notifier extension.
>>>
>>> The memfile_notifier extension provides two sets of callbacks for KVM to
>>> interact with the memory backing store:
>>>    - memfile_notifier_ops: callbacks for memory backing store to notify
>>>      KVM when memory gets invalidated.
>>>    - backing store callbacks: callbacks for KVM to call into memory
>>>      backing store to request memory pages for guest private memory.
>>>
>>> The memfile_notifier extension also provides APIs for memory backing
>>> store to register/unregister itself and to trigger the notifier when the
>>> bookmarked memory gets invalidated.
>>>
>>> The patchset also introduces a new memfd seal F_SEAL_AUTO_ALLOCATE to
>>> prevent double allocation caused by unintentional guest when we only
>>> have a single side of the shared/private memfds effective.
>>>
>>> memslot extension
>>> -----------------
>>> Add the private fd and the fd offset to existing 'shared' memslot so
>>> that both private/shared guest memory can live in one single memslot.
>>> A page in the memslot is either private or shared. Whether a guest page
>>> is private or shared is maintained through reusing existing SEV ioctls
>>> KVM_MEMORY_ENCRYPT_{UN,}REG_REGION.
>>>
>>> Test
>>> ----
>>> To test the new functionalities of this patch TDX patchset is needed.
>>> Since TDX patchset has not been merged so I did two kinds of test:
>>>
>>> -  Regresion test on kvm/queue (this patchset)
>>>     Most new code are not covered. Code also in below repo:
>>>     https://github.com/chao-p/linux/tree/privmem-v7
>>>
>>> -  New Funational test on latest TDX code
>>>     The patch is rebased to latest TDX code and tested the new
>>>     funcationalities. See below repos:
>>>     Linux: https://github.com/chao-p/linux/tree/privmem-v7-tdx
>>>     QEMU: https://github.com/chao-p/qemu/tree/privmem-v7
>>
>> While debugging an issue with SEV+UPM, found that fallocate() returns
>> an error in QEMU which is not handled (EINTR). With the below handling
>> of EINTR subsequent fallocate() succeeds:
>>
>>
>> diff --git a/backends/hostmem-memfd-private.c b/backends/hostmem-memfd-private.c
>> index af8fb0c957..e8597ed28d 100644
>> --- a/backends/hostmem-memfd-private.c
>> +++ b/backends/hostmem-memfd-private.c
>> @@ -39,7 +39,7 @@ priv_memfd_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
>>       MachineState *machine = MACHINE(qdev_get_machine());
>>       uint32_t ram_flags;
>>       char *name;
>> -    int fd, priv_fd;
>> +    int fd, priv_fd, ret;
>>         if (!backend->size) {
>>           error_setg(errp, "can't create backend with size 0");
>> @@ -65,7 +65,15 @@ priv_memfd_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
>>                                      backend->size, ram_flags, fd, 0, errp);
>>       g_free(name);
>>   -    fallocate(priv_fd, 0, 0, backend->size);
>> +again:
>> +    ret = fallocate(priv_fd, 0, 0, backend->size);
>> +    if (ret) {
>> +           perror("Fallocate failed: \n");
>> +           if (errno == EINTR)
>> +                   goto again;
>> +           else
>> +                   exit(1);
>> +    }
>>
>> However, fallocate() preallocates full guest memory before starting the guest.
>> With this behaviour guest memory is *not* demand pinned. Is there a way to
>> prevent fallocate() from reserving full guest memory?
> 
> Isn't the pinning being handled by the corresponding host memory backend with mmu > notifier and architecture support while doing the memory operations e.g page> migration and swapping/reclaim (not supported currently AFAIU). But yes, we need> to allocate entire guest memory with the new flags MEMFILE_F_{UNMOVABLE, UNRECLAIMABLE etc}.

That is correct, but the question is when does the memory allocated, as these flags are set,
memory is neither moved nor reclaimed. In current scenario, if I start a 32GB guest, all 32GB is
allocated.

Regards
Nikunj

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