linux-kernel - Re: [PATCH v5 07/22] x86/virt/tdx: Implement SEAMCALL function

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Date:   Wed, 27 Jul 2022 12:34:50 +1200
From:   Kai Huang <kai.huang@...el.com>
To:     Dave Hansen <dave.hansen@...el.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     seanjc@...gle.com, pbonzini@...hat.com, len.brown@...el.com,
        tony.luck@...el.com, rafael.j.wysocki@...el.com,
        reinette.chatre@...el.com, dan.j.williams@...el.com,
        peterz@...radead.org, ak@...ux.intel.com,
        kirill.shutemov@...ux.intel.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com,
        isaku.yamahata@...el.com
Subject: Re: [PATCH v5 07/22] x86/virt/tdx: Implement SEAMCALL function

On Thu, 2022-07-21 at 13:52 +1200, Kai Huang wrote:
> > > BUT, since as the first step, we cannot get the CMR during kernel boot (as
> > > it
> > > requires additional code to put CPU into VMX operation), I think for now
> > > we can
> > > handle ACPI memory hotplug in below way:
> > > 
> > > - For memory hot-removal, we do nothing.
> > 
> > This doesn't seem right to me.  *If* we get a known-bogus hot-remove
> > event, we need to reject it.  Remember, removal is a two-step process.
> 
> If so, we need to reject the (CMR) memory offline.  Or we just BUG() in the
> ACPI
> memory removal  callback?
> 
> But either way this will requires us to get the CMRs during kernel boot.
> 
> Do you think we need to add this support in the first series?

Hi Dave,

In terms of whether we should get CMRs during kernel boot (which requires we do
VMXON/VMXOFF during kernel boot around SEAMCALL), I forgot one thing:

Technically, ACPI memory hotplug is related to whether TDX is enabled in BIOS,
but not related to whether TDX module is loaded or not.  With doing
VMXON/VMXOFF, we can get CMRs during kernel boot by calling P-SEAMLDR's
SEAMCALL.  But theoretically, from TDX architecture's point of view, the P-
SEAMLDR may not be loaded even TDX is enabled by BIOS (in practice, the P-
SEAMLDR is always loaded by BIOS when TDX is enabled), in which case there's no
way we can get CMRs.  But in this case, I think we can just treat TDX isn't
enabled by BIOS as kernel should never try to load P-SEAMLDR.

Other advantages of being able to do VMXON/VMXOFF and getting CMRs during kernel
boot:

1) We can just shut down the TDX module in kexec();
2) We can choose to trim any non-CMR memory out of memblock.memory instead of
having to manually verify all memory regions in memblock are CMR memory.

Comments?