linux-kernel - Re: [PATCH v5 08/13] KVM: Use memfile_pfn

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Message-ID: <20220428123751.GB10508@chaop.bj.intel.com>
Date:   Thu, 28 Apr 2022 20:37:51 +0800
From:   Chao Peng <chao.p.peng@...ux.intel.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-api@...r.kernel.org, qemu-devel@...gnu.org,
        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>,
        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
Subject: Re: [PATCH v5 08/13] KVM: Use memfile_pfn_ops to obtain pfn for
 private pages

On Mon, Mar 28, 2022 at 11:56:06PM +0000, Sean Christopherson wrote:
> On Thu, Mar 10, 2022, Chao Peng wrote:
> > @@ -2217,4 +2220,34 @@ static inline void kvm_handle_signal_exit(struct kvm_vcpu *vcpu)
> >  /* Max number of entries allowed for each kvm dirty ring */
> >  #define  KVM_DIRTY_RING_MAX_ENTRIES  65536
> >  
> > +#ifdef CONFIG_MEMFILE_NOTIFIER
> > +static inline long kvm_memfile_get_pfn(struct kvm_memory_slot *slot, gfn_t gfn,
> > +				       int *order)
> > +{
> > +	pgoff_t index = gfn - slot->base_gfn +
> > +			(slot->private_offset >> PAGE_SHIFT);
> 
> This is broken for 32-bit kernels, where gfn_t is a 64-bit value but pgoff_t is a
> 32-bit value.  There's no reason to support this for 32-bit kernels, so...
> 
> The easiest fix, and likely most maintainable for other code too, would be to
> add a dedicated CONFIG for private memory, and then have KVM check that for all
> the memfile stuff.  x86 can then select it only for 64-bit kernels, and in turn
> select MEMFILE_NOTIFIER iff private memory is supported.
> 
> diff --git a/arch/x86/kvm/Kconfig b/arch/x86/kvm/Kconfig
> index ca7b2a6a452a..ee9c8c155300 100644
> --- a/arch/x86/kvm/Kconfig
> +++ b/arch/x86/kvm/Kconfig
> @@ -48,7 +48,9 @@ config KVM
>         select SRCU
>         select INTERVAL_TREE
>         select HAVE_KVM_PM_NOTIFIER if PM
> -       select MEMFILE_NOTIFIER
> +       select HAVE_KVM_PRIVATE_MEM if X86_64
> +       select MEMFILE_NOTIFIER if HAVE_KVM_PRIVATE_MEM
> +
>         help
>           Support hosting fully virtualized guest machines using hardware
>           virtualization extensions.  You will need a fairly recent
> 
> And in addition to replacing checks on CONFIG_MEMFILE_NOTIFIER, the probing of
> whether or not KVM_MEM_PRIVATE is allowed can be:
> 
> @@ -1499,23 +1499,19 @@ static void kvm_replace_memslot(struct kvm *kvm,
>         }
>  }
> 
> -bool __weak kvm_arch_private_memory_supported(struct kvm *kvm)
> -{
> -       return false;
> -}
> -
>  static int check_memory_region_flags(struct kvm *kvm,
>                                 const struct kvm_userspace_memory_region *mem)
>  {
>         u32 valid_flags = KVM_MEM_LOG_DIRTY_PAGES;
> 
> -       if (kvm_arch_private_memory_supported(kvm))
> -               valid_flags |= KVM_MEM_PRIVATE;
> -
>  #ifdef __KVM_HAVE_READONLY_MEM
>         valid_flags |= KVM_MEM_READONLY;
>  #endif
> 
> +#ifdef CONFIG_KVM_HAVE_PRIVATE_MEM
> +       valid_flags |= KVM_MEM_PRIVATE;
> +#endif

One thing to mention is CONFIG_KVM_HAVE_PRIVATE_MEM is build-time thing.
Do you think we should or not do that for runtime? E.g. expose by vm_type
so only when TDX is enabled KVM_MEM_PRIVATE is exposed.

Chao