Re: [PATCH v19 056/130] KVM: x86/tdp_mmu: Init role member of struct kvm_mmu_page at allocation

[Isaku Yamahata]

On Fri, Mar 22, 2024 at 03:18:39PM +0800,
Chao Gao <chao.gao@xxxxxxxxx> wrote:

> On Thu, Mar 21, 2024 at 02:24:12PM -0700, Isaku Yamahata wrote:
> >On Thu, Mar 21, 2024 at 12:11:11AM +0000,
> >"Edgecombe, Rick P" <rick.p.edgecombe@xxxxxxxxx> wrote:
> >
> >> On Mon, 2024-02-26 at 00:25 -0800, isaku.yamahata@xxxxxxxxx wrote:
> >> > To handle private page tables, argument of is_private needs to be
> >> > passed
> >> > down.  Given that already page level is passed down, it would be
> >> > cumbersome
> >> > to add one more parameter about sp. Instead replace the level
> >> > argument with
> >> > union kvm_mmu_page_role.  Thus the number of argument won't be
> >> > increased
> >> > and more info about sp can be passed down.
> >> > 
> >> > For private sp, secure page table will be also allocated in addition
> >> > to
> >> > struct kvm_mmu_page and page table (spt member).  The allocation
> >> > functions
> >> > (tdp_mmu_alloc_sp() and __tdp_mmu_alloc_sp_for_split()) need to know
> >> > if the
> >> > allocation is for the conventional page table or private page table. 
> >> > Pass
> >> > union kvm_mmu_role to those functions and initialize role member of
> >> > struct
> >> > kvm_mmu_page.
> >> 
> >> tdp_mmu_alloc_sp() is only called in two places. One for the root, and
> >> one for the mid-level tables.
> >> 
> >> In later patches when the kvm_mmu_alloc_private_spt() part is added,
> >> the root case doesn't need anything done. So the code has to take
> >> special care in tdp_mmu_alloc_sp() to avoid doing anything for the
> >> root.
> >> 
> >> It only needs to do the special private spt allocation in non-root
> >> case. If we open code that case, I think maybe we could drop this
> >> patch, like the below.
> >> 
> >> The benefits are to drop this patch (which looks to already be part of
> >> Paolo's series), and simplify "KVM: x86/mmu: Add a private pointer to
> >> struct kvm_mmu_page". I'm not sure though, what do you think? Only
> >> build tested.
> >
> >Makes sense.  Until v18, it had config to disable private mmu part at
> >compile time.  Those functions have #ifdef in mmu_internal.h.  v19
> >dropped the config for the feedback.
> >  https://lore.kernel.org/kvm/Zcrarct88veirZx7@xxxxxxxxxx/
> >
> >After looking at mmu_internal.h, I think the following three function could be
> >open coded.
> >kvm_mmu_private_spt(), kvm_mmu_init_private_spt(), kvm_mmu_alloc_private_spt(),
> >and kvm_mmu_free_private_spt().
> 
> It took me a few minutes to figure out why the mirror root page doesn't need
> a private_spt.
> 
> Per TDX module spec:
> 
>   Secure EPT’s root page (EPML4 or EPML5, depending on whether the host VMM uses
>   4-level or 5-level EPT) does not need to be explicitly added. It is created
>   during TD initialization (TDH.MNG.INIT) and is stored as part of TDCS.
> 
> I suggest adding the above as a comment somewhere even if we decide to open-code
> kvm_mmu_alloc_private_spt().


058/130 has such comment.  The citation from the spec would be better.



> IMO, some TDX details bleed into KVM MMU regardless of whether we open-code
> kvm_mmu_alloc_private_spt() or not. This isn't good though I cannot think of
> a better solution.
> 

-- 
Isaku Yamahata <isaku.yamahata@xxxxxxxxx>