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

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Date:   Wed, 24 May 2023 02:28:51 +0300
From:   kirill.shutemov@...ux.intel.com
To:     Dave Hansen <dave.hansen@...el.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,
        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, seanjc@...gle.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
Subject: Re: [PATCH v6 2/6] x86/tdx: Support vmalloc() for
 tdx_enc_status_changed()

On Tue, May 23, 2023 at 03:43:15PM -0700, Dave Hansen wrote:
> On 5/23/23 15:37, kirill.shutemov@...ux.intel.com wrote:
> >> 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?
> > Alias processing in __change_page_attr_set_clr() will change direct
> > mapping if you call it on vmalloc()ed memory. I think we are safe wrt
> > load_unaligned_zeropad() here.
> 
> We're *eventually* OK:
> 
> >         /* Notify hypervisor that we are about to set/clr encryption attribute. */
> >         x86_platform.guest.enc_status_change_prepare(addr, numpages, enc);
> > 
> >         ret = __change_page_attr_set_clr(&cpa, 1);
> 
> But what about in the middle between enc_status_change_prepare() and
> __change_page_attr_set_clr()?  Don't the direct map and the
> shared/private status of the page diverge in there?

Hmm. Maybe we would need to go through making the range in direct mapping
non-present before notifying VMM about the change.

I need to look at this again in the morning.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov