Re: [PATCH v18 060/121] KVM: TDX: TDP MMU TDX support

[Isaku Yamahata]

On Tue, Jan 30, 2024 at 11:31:22PM +0800,
Binbin Wu <binbin.wu@xxxxxxxxxxxxxxx> wrote:

> > +
> > +/*
> > + * TLB shoot down procedure:
> > + * There is a global epoch counter and each vcpu has local epoch counter.
> > + * - TDH.MEM.RANGE.BLOCK(TDR. level, range) on one vcpu
> > + *   This blocks the subsequenct creation of TLB translation on that range.
> > + *   This corresponds to clear the present bit(all RXW) in EPT entry
> > + * - TDH.MEM.TRACK(TDR): advances the epoch counter which is global.
> > + * - IPI to remote vcpus
> > + * - TDExit and re-entry with TDH.VP.ENTER on remote vcpus
> > + * - On re-entry, TDX module compares the local epoch counter with the global
> > + *   epoch counter.  If the local epoch counter is older than the global epoch
> > + *   counter, update the local epoch counter and flushes TLB.
> > + */
> > +static void tdx_track(struct kvm *kvm)
> > +{
> > +	struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm);
> > +	u64 err;
> > +
> > +	KVM_BUG_ON(!is_hkid_assigned(kvm_tdx), kvm);
> > +	/* If TD isn't finalized, it's before any vcpu running. */
> > +	if (unlikely(!is_td_finalized(kvm_tdx)))
> > +		return;
> > +
> > +	/*
> > +	 * tdx_flush_tlb() waits for this function to issue TDH.MEM.TRACK() by
> > +	 * the counter.  The counter is used instead of bool because multiple
> > +	 * TDH_MEM_TRACK() can be issued concurrently by multiple vcpus.
> > +	 */
> > +	atomic_inc(&kvm_tdx->tdh_mem_track);
> > +	/*
> > +	 * KVM_REQ_TLB_FLUSH waits for the empty IPI handler, ack_flush(), with
> > +	 * KVM_REQUEST_WAIT.
> > +	 */
> > +	kvm_make_all_cpus_request(kvm, KVM_REQ_TLB_FLUSH);
> > +
> > +	do {
> > +		/*
> > +		 * kvm_flush_remote_tlbs() doesn't allow to return error and
> > +		 * retry.
> > +		 */
> > +		err = tdh_mem_track(kvm_tdx->tdr_pa);
> > +	} while (unlikely((err & TDX_SEAMCALL_STATUS_MASK) == TDX_OPERAND_BUSY));
> 
> Why the sequence of the code is different from the description of the
> function.
> In the description, do the TDH.MEM.TRACK before IPIs.
> But in the code, do TDH.MEM.TRACK after IPIs?

It's intentional to handle IPI in parallel as we already introduced
tdh_mem_track.
-- 
Isaku Yamahata <isaku.yamahata@xxxxxxxxxxxxxxx>