linux-kernel - Re: [PATCH v2 00/25] TDX vCPU/VM creation

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Message-ID: <ZzMmomR-orhes_-p@tlindgre-MOBL1>
Date: Tue, 12 Nov 2024 11:57:54 +0200
From: Tony Lindgren <tony.lindgren@...ux.intel.com>
To: Adrian Hunter <adrian.hunter@...el.com>
Cc: Rick Edgecombe <rick.p.edgecombe@...el.com>, pbonzini@...hat.com,
	seanjc@...gle.com, yan.y.zhao@...el.com, isaku.yamahata@...il.com,
	kai.huang@...el.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, xiaoyao.li@...el.com,
	reinette.chatre@...el.com
Subject: Re: [PATCH v2 00/25] TDX vCPU/VM creation

On Tue, Nov 12, 2024 at 09:26:36AM +0200, Adrian Hunter wrote:
> On 11/11/24 11:49, Tony Lindgren wrote:
> > On Thu, Oct 31, 2024 at 09:21:29PM +0200, Adrian Hunter wrote:
> >> On 30/10/24 21:00, Rick Edgecombe wrote:
> >>> Here is v2 of TDX VM/vCPU creation series. As discussed earlier, non-nits 
> >>> from v1[0] have been applied and it’s ready to hand off to Paolo. A few 
> >>> items remain that may be worth further discussion:
> >>>  - Disable CET/PT in tdx_get_supported_xfam(), as these features haven’t 
> >>>    been been tested.
> >>
> >> It seems for Intel PT we have no support for restoring host
> >> state.  IA32_RTIT_* MSR preservation is Init(XFAM(8)) which means
> >> the TDX Module sets the MSR to its RESET value after TD Enty/Exit.
> >> So it seems to me XFAM(8) does need to be disabled until that is
> >> supported.
> > 
> > So for now, we should remove the PT bit from tdx_get_supported_xfam(),
> > but can still keep it in tdx_restore_host_xsave_state()?
> 
> Yes
> 
> > 
> > Then for save/restore, maybe we can just use the pt_guest_enter() and
> > pt_guest_exit() also for TDX. Some additional checks are needed for
> > the pt_mode though as the TDX module always clears the state if PT is
> > enabled. And the PT_MODE_SYSTEM will be missing TDX enter/exit data
> > but might be otherwise usable.
> 
> pt_guest_enter() / pt_guest_exit() are not suitable for TDX.  pt_mode
> is not relevant for TDX because the TDX guest is always hidden from the
> host behind SEAM.  However, restoring host MSRs is not the only issue.
> 
> The TDX Module does not validate Intel PT CPUID leaf 0x14
> (except it must be all zero if Intel PT is not supported
> i.e. if XFAM bit 8 is zero).  For invalid MSR accesses by the guest,
> the TDX Module will inject #GP.  Host VMM could provide valid CPUID
> to avoid that, but it would also need to be valid for the destination
> platform if migration was to be attempted.
> 
> Disabling Intel PT for TDX for now also avoids that issue.

OK thanks for the detailed explanation.

Regards,

Tony