linux-kernel - Re: [PATCH v6 2/6] x86/tdx: Support vmalloc() for tdx_enc_status

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Date:   Tue, 23 May 2023 14:33:02 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Dexuan Cui <decui@...rosoft.com>, ak@...ux.intel.com,
        arnd@...db.de, bp@...en8.de, brijesh.singh@....com,
        dan.j.williams@...el.com, dave.hansen@...ux.intel.com,
        haiyangz@...rosoft.com, hpa@...or.com, jane.chu@...cle.com,
        kirill.shutemov@...ux.intel.com, kys@...rosoft.com,
        linux-arch@...r.kernel.org, linux-hyperv@...r.kernel.org,
        luto@...nel.org, mingo@...hat.com, peterz@...radead.org,
        rostedt@...dmis.org, sathyanarayanan.kuppuswamy@...ux.intel.com,
        tglx@...utronix.de, tony.luck@...el.com, wei.liu@...nel.org,
        x86@...nel.org, mikelley@...rosoft.com,
        linux-kernel@...r.kernel.org, Tianyu.Lan@...rosoft.com,
        "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
Subject: Re: [PATCH v6 2/6] x86/tdx: Support vmalloc() for
 tdx_enc_status_changed()

On 5/23/23 14:25, Sean Christopherson wrote:
>> 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.

I don't think we want a GFP flag.  This is still way too specialized to
warrant one of those.

It sounds like a similar problem to what folks want for modules or BPF.
There are a bunch of allocations that are related and can have some of
their setup/teardown costs amortized if they can be clumped together.

For BPF, the costs are from doing RW=>RO in the kernel direct map, and
fracturing it in the process.

Here, the costs are from the private->shared conversions and fracturing
both the direct map and the EPT/SEPT.

I just don't know if there's anything that we can reuse from the BPF effort.