Re: [PATCH v19 039/130] KVM: TDX: initialize VM with TDX specific parameters

[Isaku Yamahata]

On Mon, Mar 25, 2024 at 10:39:10AM +0000,
"Huang, Kai" <kai.huang@xxxxxxxxx> wrote:

> On Fri, 2024-03-22 at 18:22 -0700, Yamahata, Isaku wrote:
> > On Fri, Mar 22, 2024 at 11:20:01AM +0000,
> > "Huang, Kai" <kai.huang@xxxxxxxxx> wrote:
> > 
> > > On Mon, 2024-02-26 at 00:25 -0800, isaku.yamahata@xxxxxxxxx wrote:
> > > > +struct kvm_tdx_init_vm {
> > > > +	__u64 attributes;
> > > > +	__u64 mrconfigid[6];	/* sha384 digest */
> > > > +	__u64 mrowner[6];	/* sha384 digest */
> > > > +	__u64 mrownerconfig[6];	/* sha384 digest */
> > > > +	/*
> > > > +	 * For future extensibility to make sizeof(struct kvm_tdx_init_vm) = 8KB.
> > > > +	 * This should be enough given sizeof(TD_PARAMS) = 1024.
> > > > +	 * 8KB was chosen given because
> > > > +	 * sizeof(struct kvm_cpuid_entry2) * KVM_MAX_CPUID_ENTRIES(=256) = 8KB.
> > > > +	 */
> > > > +	__u64 reserved[1004];
> > > 
> > > This is insane.
> > > 
> > > You said you want to reserve 8K for CPUID entries, but how can these 1004 * 8
> > > bytes be used for CPUID entries since ...
> > 
> > I tried to overestimate it. It's too much, how about to make it
> > 1024, reserved[109]?
> > 
> 
> I am not sure why we need 1024B either.
> 
> IIUC, the inputs here in 'kvm_tdx_init_vm' should be a subset of the members in
> TD_PARAMS.  This IOCTL() isn't intended to carry any additional input besides
> these defined in TD_PARAMS, right?
> 
> If so, then it seems to me you "at most" only need to reserve the space for the
> members excluding the CPUID entries, because for the CPUID entries we will
> always pass them as a flexible array at the end of the structure.
> 
> Based on the spec, the "non-CPUID-entry" part only occupies 256 bytes.  To me it
> seems we have no reason to reserve more space than 256 bytes.

Ok, I'll make it 256 bytes.

The alternative is to use key-value.  The user space loops to set all necessary
parameters.  Something like as follows.

KVM_TDX_SET_VM_PARAM

struct kvm_tdx_vm_param {
        /* TDCS metadata field. */
        __u64 field_id;
        /*
         * value for attributes or data less or qeual to __u64.
         * pointer for sha384, cpuid, or data larger than __u64.
         */
        __u64 value_or_ptr;
};
-- 
Isaku Yamahata <isaku.yamahata@xxxxxxxxx>