linux-kernel - Re: [PATCH v8 020/103] KVM: TDX: create/destroy VM structure

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Date:   Thu, 25 Aug 2022 23:44:18 -0700
From:   Isaku Yamahata <isaku.yamahata@...il.com>
To:     Erdem Aktas <erdemaktas@...gle.com>
Cc:     "Yamahata, Isaku" <isaku.yamahata@...el.com>,
        "open list:KERNEL VIRTUAL MACHINE (KVM)" <kvm@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        Isaku Yamahata <isaku.yamahata@...il.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Sagi Shahar <sagis@...gle.com>
Subject: Re: [PATCH v8 020/103] KVM: TDX: create/destroy VM structure

On Tue, Aug 23, 2022 at 05:53:00PM -0700,
Erdem Aktas <erdemaktas@...gle.com> wrote:

> On Sun, Aug 7, 2022 at 3:03 PM <isaku.yamahata@...el.com> wrote:

> > +static void tdx_clear_page(unsigned long page)
> > +{
> > +       const void *zero_page = (const void *) __va(page_to_phys(ZERO_PAGE(0)));
> > +       unsigned long i;
> > +
> > +       /*
> > +        * Zeroing the page is only necessary for systems with MKTME-i:
> > +        * when re-assign one page from old keyid to a new keyid, MOVDIR64B is
> > +        * required to clear/write the page with new keyid to prevent integrity
> > +        * error when read on the page with new keyid.
> > +        */
> 
> Are we saying that we do not need to use MOVDIR64B to clear pages with Li?

Yes. TDX module spec, Table 15.3: Checks on Memory Reads in Li mode says that
read with shared HKID doesn't result in #MC.  Read of poisoned cache line is
another story, though.


> > +       if (!static_cpu_has(X86_FEATURE_MOVDIR64B))
> > +               return;
> > +
> > +       for (i = 0; i < 4096; i += 64)
> > +               /* MOVDIR64B [rdx], es:rdi */
> > +               asm (".byte 0x66, 0x0f, 0x38, 0xf8, 0x3a"
> > +                    : : "d" (zero_page), "D" (page + i) : "memory");
> > +}
> 
> According to the Software Developer Manual, mfence is required for a
> strong ordering on stores. Should we not use mfence here?

Right. I'll add __mb(). thanks for catching it.

-- 
Isaku Yamahata <isaku.yamahata@...il.com>