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Date:   Tue, 23 May 2023 14:25:50 -0700
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Subject: Re: [PATCH v6 2/6] x86/tdx: Support vmalloc() for tdx_enc_status_changed()
From:   Sean Christopherson <seanjc@google.com>
To:     Dave Hansen <dave.hansen@intel.com>
Cc:     Dexuan Cui <decui@microsoft.com>, ak@linux.intel.com,
        arnd@arndb.de, bp@alien8.de, brijesh.singh@amd.com,
        dan.j.williams@intel.com, dave.hansen@linux.intel.com,
        haiyangz@microsoft.com, hpa@zytor.com, jane.chu@oracle.com,
        kirill.shutemov@linux.intel.com, kys@microsoft.com,
        linux-arch@vger.kernel.org, linux-hyperv@vger.kernel.org,
        luto@kernel.org, mingo@redhat.com, peterz@infradead.org,
        rostedt@goodmis.org, sathyanarayanan.kuppuswamy@linux.intel.com,
        tglx@linutronix.de, tony.luck@intel.com, wei.liu@kernel.org,
        x86@kernel.org, mikelley@microsoft.com,
        linux-kernel@vger.kernel.org, Tianyu.Lan@microsoft.com
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On Tue, May 23, 2023, Dave Hansen wrote:
> On 5/4/23 15:53, Dexuan Cui wrote:
> > When a TDX guest runs on Hyper-V, the hv_netvsc driver's netvsc_init_buf()
> > allocates buffers using vzalloc(), and needs to share the buffers with the
> > host OS by calling set_memory_decrypted(), which is not working for
> > vmalloc() yet. Add the support by handling the pages one by one.
> 
> I think this sets a bad precedent.

+1 

> There are consequences for converting pages between shared and private.
> Doing it on a vmalloc() mapping is guaranteed to fracture the underlying
> EPT/SEPT mappings.
> 
> How does this work with load_unaligned_zeropad()?  Couldn't it be
> running around poking at one of these vmalloc()'d pages via the direct
> map during a shared->private conversion before the page has been accepted?

Would it be feasible and sensible to add a GFP_SHARED or whatever, to communicate
to the core allocators that the page is destined to be converted to a shared page?
I assume that would provide a common place (or two) for initiating conversions,
and would hopefully allow for future optimizations, e.g. to keep shared allocation
in the same pool or whatever.  Sharing memory without any intelligence as to what
memory is converted is going to make both the guest and host sad.