Re: [PATCH v19 027/130] KVM: TDX: Define TDX architectural definitions

[Huang, Kai]

On 17/04/2024 4:28 am, Yamahata, Isaku wrote:
> On Tue, Apr 16, 2024 at 12:55:33PM +1200,
> "Huang, Kai" <kai.huang@xxxxxxxxx> wrote:
>
> > On 5/03/2024 9:21 pm, Isaku Yamahata wrote:
> > > On Fri, Mar 01, 2024 at 03:25:31PM +0800,
> > > Yan Zhao <yan.y.zhao@xxxxxxxxx> wrote:
> > >
> > > > > + * TD_PARAMS is provided as an input to TDH_MNG_INIT, the size of which is 1024B.
> > > > > + */
> > > > > +#define TDX_MAX_VCPUS	(~(u16)0)
> > > > This value will be treated as -1 in tdx_vm_init(),
> > > > 	"kvm->max_vcpus = min(kvm->max_vcpus, TDX_MAX_VCPUS);"
> > > >
> > > > This will lead to kvm->max_vcpus being -1 by default.
> > > > Is this by design or just an error?
> > > > If it's by design, why not set kvm->max_vcpus = -1 in tdx_vm_init() directly.
> > > > If an unexpected error, may below is better?
> > > >
> > > > #define TDX_MAX_VCPUS   (int)((u16)(~0UL))
> > > > or
> > > > #define TDX_MAX_VCPUS 65536
> > >
> > > You're right. I'll use ((int)U16_MAX).
> > > As TDX 1.5 introduced metadata MAX_VCPUS_PER_TD, I'll update to get the value
> > > and trim it further. Something following.
> >
> > [...]
> >
> > > +	u16 max_vcpus_per_td;
> > > +
> >
> > [...]
> >
> > > -	kvm->max_vcpus = min(kvm->max_vcpus, TDX_MAX_VCPUS);
> > > +	kvm->max_vcpus = min3(kvm->max_vcpus, tdx_info->max_vcpus_per_td,
> > > +			     TDX_MAX_VCPUS);
> >
> > [...]
> >
> > > -#define TDX_MAX_VCPUS	(~(u16)0)
> > > +#define TDX_MAX_VCPUS	((int)U16_MAX)
> >
> > Why do you even need TDX_MAX_VCPUS, given it cannot exceed U16_MAX and you
> > will have the 'u16 max_vcpus_per_td' anyway?
> >
> > IIUC, in KVM_ENABLE_CAP(KVM_CAP_MAX_VCPUS), we can overwrite the
> > kvm->max_vcpus to the 'max_vcpus' provided by the userspace, and make sure
> > it doesn't exceed tdx_info->max_vcpus_per_td.
> >
> > Anything I am missing?
>
> With the latest TDX 1.5 module, we don't need TDX_MAX_VCPUS.
>
> The metadata MD_FIELD_ID_MAX_VCPUS_PER_TD was introduced at the middle version
> of TDX 1.5. (I don't remember the exact version.), the logic was something
> like as follows.  Now if we fail to read the metadata, disable TDX.
>
> read metadata MD_FIELD_ID_MAX_VCPUS_PER_TD;
> if success
>    tdx_info->max_vcpu_per_td = the value read metadata
> else
>    tdx_info->max_vcpu_per_td = TDX_MAX_VCPUS;

OK.  But even the SEAMCALL can fail, we can just use U16_MAX directly 
when it fails given we can see clearly the type of max_vcpu_per_td is 'u16'.

if success
	tdx_info->max_vcpu_per_td = the value read metadata
else
	tdx_info->max_vcpu_per_td = U16_MAX;

So I don't see why TDX_MAX_VCPUS is needed (especially in tdx_arch.h as 
it is not an architectural value but just some value chosen for our 
convenience).