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Date: Wed, 3 Apr 2024 10:50:56 -0700
From: Isaku Yamahata <isaku.yamahata@intel.com>
To: Binbin Wu <binbin.wu@linux.intel.com>
Cc: isaku.yamahata@intel.com, kvm@vger.kernel.org,
	linux-kernel@vger.kernel.org, isaku.yamahata@gmail.com,
	Paolo Bonzini <pbonzini@redhat.com>, erdemaktas@google.com,
	Sean Christopherson <seanjc@google.com>,
	Sagi Shahar <sagis@google.com>, Kai Huang <kai.huang@intel.com>,
	chen.bo@intel.com, hang.yuan@intel.com, tina.zhang@intel.com,
	isaku.yamahata@linux.intel.com
Subject: Re: [PATCH v19 070/130] KVM: TDX: TDP MMU TDX support
Message-ID: <20240403175056.GG2444378@ls.amr.corp.intel.com>
References: <cover.1708933498.git.isaku.yamahata@intel.com>
 <56cdb0da8bbf17dc293a2a6b4ff74f6e3e034bbd.1708933498.git.isaku.yamahata@intel.com>
 <bd862ee4-7513-4880-b6e5-466dcfb08f1a@linux.intel.com>
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On Tue, Apr 02, 2024 at 02:21:41PM +0800,
Binbin Wu <binbin.wu@linux.intel.com> wrote:

> 
> 
> On 2/26/2024 4:26 PM, isaku.yamahata@intel.com wrote:
> > From: Isaku Yamahata <isaku.yamahata@intel.com>
> > 
> > Implement hooks of TDP MMU for TDX backend.  TLB flush, TLB shootdown,
> > propagating the change private EPT entry to Secure EPT and freeing Secure
> > EPT page. TLB flush handles both shared EPT and private EPT.  It flushes
> > shared EPT same as VMX.  It also waits for the TDX TLB shootdown.  For the
> > hook to free Secure EPT page, unlinks the Secure EPT page from the Secure
> > EPT so that the page can be freed to OS.
> > 
> > Propagate the entry change to Secure EPT.  The possible entry changes are
> > present -> non-present(zapping) and non-present -> present(population).  On
> > population just link the Secure EPT page or the private guest page to the
> > Secure EPT by TDX SEAMCALL. Because TDP MMU allows concurrent
> > zapping/population, zapping requires synchronous TLB shoot down with the
> > frozen EPT entry.
> 
> But for private memory, zapping holds write lock, right?

Right.


> >    It zaps the secure entry, increments TLB counter, sends
> > IPI to remote vcpus to trigger TLB flush, and then unlinks the private
> > guest page from the Secure EPT. For simplicity, batched zapping with
> > exclude lock is handled as concurrent zapping.
> 
> exclude lock -> exclusive lock
> 
> How to understand this sentence?
> Since it's holding exclusive lock, how it can be handled as concurrent
> zapping?
> Or you want to describe the current implementation prevents concurrent
> zapping?

The sentences is mixture of the currenct TDP MMU and how the new enhancement
with this patch provides.  Because this patch is TDX backend, let me drop the
description about the TDP MMU part.

Propagate the entry change to Secure EPT.  The possible entry changes
are non-present -> present(population) and present ->
non-present(zapping).  On population just link the Secure EPT page or
the private guest page to the Secure EPT by TDX SEAMCALL. On zapping,
It blocks the Secure-EPT entry (clear present bit) , increments TLB
counter, sends IPI to remote vcpus to trigger TLB flush, and then
unlinks the private guest page from the Secure EPT.


> > Although it's inefficient,
> > it can be optimized in the future.
> > 
> > For MMIO SPTE, the spte value changes as follows.
> > initial value (suppress VE bit is set)
> > -> Guest issues MMIO and triggers EPT violation
> > -> KVM updates SPTE value to MMIO value (suppress VE bit is cleared)
> > -> Guest MMIO resumes.  It triggers VE exception in guest TD
> > -> Guest VE handler issues TDG.VP.VMCALL<MMIO>
> > -> KVM handles MMIO
> > -> Guest VE handler resumes its execution after MMIO instruction
> > 
> > Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
> > 
> > ---
> > v19:
> > - Compile fix when CONFIG_HYPERV != y.
> >    It's due to the following patch.  Catch it up.
> >    https://lore.kernel.org/all/20231018192325.1893896-1-seanjc@google.com/
> > - Add comments on tlb shootdown to explan the sequence.
> > - Use gmem_max_level callback, delete tdp_max_page_level.
> > 
> > v18:
> > - rename tdx_sept_page_aug() -> tdx_mem_page_aug()
> > - checkpatch: space => tab
> > 
> > v15 -> v16:
> > - Add the handling of TD_ATTR_SEPT_VE_DISABLE case.
> > 
> > v14 -> v15:
> > - Implemented tdx_flush_tlb_current()
> > - Removed unnecessary invept in tdx_flush_tlb().  It was carry over
> >    from the very old code base.
> > 
> > Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
> > ---
> >   arch/x86/kvm/mmu/spte.c    |   3 +-
> >   arch/x86/kvm/vmx/main.c    |  91 ++++++++-
> >   arch/x86/kvm/vmx/tdx.c     | 372 +++++++++++++++++++++++++++++++++++++
> >   arch/x86/kvm/vmx/tdx.h     |   2 +-
> >   arch/x86/kvm/vmx/tdx_ops.h |   6 +
> >   arch/x86/kvm/vmx/x86_ops.h |  13 ++
> >   6 files changed, 481 insertions(+), 6 deletions(-)
> > 
> [...]
> > +
> > +static int tdx_mem_page_aug(struct kvm *kvm, gfn_t gfn,
> > +			    enum pg_level level, kvm_pfn_t pfn)
> > +{
> > +	int tdx_level = pg_level_to_tdx_sept_level(level);
> > +	struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm);
> > +	union tdx_sept_level_state level_state;
> > +	hpa_t hpa = pfn_to_hpa(pfn);
> > +	gpa_t gpa = gfn_to_gpa(gfn);
> > +	struct tdx_module_args out;
> > +	union tdx_sept_entry entry;
> > +	u64 err;
> > +
> > +	err = tdh_mem_page_aug(kvm_tdx->tdr_pa, gpa, hpa, &out);
> > +	if (unlikely(err == TDX_ERROR_SEPT_BUSY)) {
> > +		tdx_unpin(kvm, pfn);
> > +		return -EAGAIN;
> > +	}
> > +	if (unlikely(err == (TDX_EPT_ENTRY_STATE_INCORRECT | TDX_OPERAND_ID_RCX))) {
> > +		entry.raw = out.rcx;
> > +		level_state.raw = out.rdx;
> > +		if (level_state.level == tdx_level &&
> > +		    level_state.state == TDX_SEPT_PENDING &&
> > +		    entry.leaf && entry.pfn == pfn && entry.sve) {
> > +			tdx_unpin(kvm, pfn);
> > +			WARN_ON_ONCE(!(to_kvm_tdx(kvm)->attributes &
> > +				       TDX_TD_ATTR_SEPT_VE_DISABLE));
> 
> to_kvm_tdx(kvm) -> kvm_tdx
> 
> Since the implementation requires attributes.TDX_TD_ATTR_SEPT_VE_DISABLE is set,

TDX KVM allows either configuration. set or cleared.


> should it check the value passed from userspace?

It's user-space configurable value.


> And the reason should be described somewhere in changelog or/and comment.

This WARN_ON_ONCE() is a guard for buggy TDX module. It shouldn't return
(TDX_EPT_ENTRY_STATE_INCORRECT | TDX_OPERAND_ID_RCX)) when SEPT_VE_DISABLED
cleared.  Maybe we should remove this WARN_ON_ONCE() because the TDX module
is mature.


> > +			return -EAGAIN;
> > +		}
> > +	}
> > +	if (KVM_BUG_ON(err, kvm)) {
> > +		pr_tdx_error(TDH_MEM_PAGE_AUG, err, &out);
> > +		tdx_unpin(kvm, pfn);
> > +		return -EIO;
> > +	}
> > +
> > +	return 0;
> > +}
> > +
> > +static int tdx_sept_set_private_spte(struct kvm *kvm, gfn_t gfn,
> > +				     enum pg_level level, kvm_pfn_t pfn)
> > +{
> > +	struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm);
> > +
> > +	/* TODO: handle large pages. */
> > +	if (KVM_BUG_ON(level != PG_LEVEL_4K, kvm))
> > +		return -EINVAL;
> > +
> > +	/*
> > +	 * Because restricted mem
> 
> The term "restricted mem" is not used anymore, right? Should update the
> comment.

Sure, will update it to guest_memfd.
> 
> > doesn't support page migration with
> > +	 * a_ops->migrate_page (yet), no callback isn't triggered for KVM on
> 
> no callback isn't -> no callback is
> 
> > +	 * page migration.  Until restricted mem supports page migration,
> 
> "restricted mem" -> guest_mem
> 
> 
> > +	 * prevent page migration.
> > +	 * TODO: Once restricted mem introduces callback on page migration,
> 
> ditto
> 
> > +	 * implement it and remove get_page/put_page().
> > +	 */
> > +	get_page(pfn_to_page(pfn));
> > +
> > +	if (likely(is_td_finalized(kvm_tdx)))
> > +		return tdx_mem_page_aug(kvm, gfn, level, pfn);
> > +
> > +	/* TODO: tdh_mem_page_add() comes here for the initial memory. */
> > +
> > +	return 0;
> > +}
> > +
> > +static int tdx_sept_drop_private_spte(struct kvm *kvm, gfn_t gfn,
> > +				       enum pg_level level, kvm_pfn_t pfn)
> > +{
> > +	int tdx_level = pg_level_to_tdx_sept_level(level);
> > +	struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm);
> > +	struct tdx_module_args out;
> > +	gpa_t gpa = gfn_to_gpa(gfn);
> > +	hpa_t hpa = pfn_to_hpa(pfn);
> > +	hpa_t hpa_with_hkid;
> > +	u64 err;
> > +
> > +	/* TODO: handle large pages. */
> > +	if (KVM_BUG_ON(level != PG_LEVEL_4K, kvm))
> > +		return -EINVAL;
> > +
> > +	if (unlikely(!is_hkid_assigned(kvm_tdx))) {
> > +		/*
> > +		 * The HKID assigned to this TD was already freed and cache
> > +		 * was already flushed. We don't have to flush again.
> > +		 */
> > +		err = tdx_reclaim_page(hpa);
> > +		if (KVM_BUG_ON(err, kvm))
> > +			return -EIO;
> > +		tdx_unpin(kvm, pfn);
> > +		return 0;
> > +	}
> > +
> > +	do {
> > +		/*
> > +		 * When zapping private page, write lock is held. So no race
> > +		 * condition with other vcpu sept operation.  Race only with
> > +		 * TDH.VP.ENTER.
> > +		 */
> > +		err = tdh_mem_page_remove(kvm_tdx->tdr_pa, gpa, tdx_level, &out);
> > +	} while (unlikely(err == TDX_ERROR_SEPT_BUSY));
> > +	if (KVM_BUG_ON(err, kvm)) {
> > +		pr_tdx_error(TDH_MEM_PAGE_REMOVE, err, &out);
> > +		return -EIO;
> > +	}
> > +
> > +	hpa_with_hkid = set_hkid_to_hpa(hpa, (u16)kvm_tdx->hkid);
> > +	do {
> > +		/*
> > +		 * TDX_OPERAND_BUSY can happen on locking PAMT entry.  Because
> > +		 * this page was removed above, other thread shouldn't be
> > +		 * repeatedly operating on this page.  Just retry loop.
> > +		 */
> > +		err = tdh_phymem_page_wbinvd(hpa_with_hkid);
> > +	} while (unlikely(err == (TDX_OPERAND_BUSY | TDX_OPERAND_ID_RCX)));
> > +	if (KVM_BUG_ON(err, kvm)) {
> > +		pr_tdx_error(TDH_PHYMEM_PAGE_WBINVD, err, NULL);
> > +		return -EIO;
> > +	}
> > +	tdx_clear_page(hpa);
> > +	tdx_unpin(kvm, pfn);
> > +	return 0;
> > +}
> > +
> > +static int tdx_sept_link_private_spt(struct kvm *kvm, gfn_t gfn,
> > +				     enum pg_level level, void *private_spt)
> > +{
> > +	int tdx_level = pg_level_to_tdx_sept_level(level);
> > +	struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm);
> > +	gpa_t gpa = gfn_to_gpa(gfn);
> > +	hpa_t hpa = __pa(private_spt);
> > +	struct tdx_module_args out;
> > +	u64 err;
> > +
> > +	err = tdh_mem_sept_add(kvm_tdx->tdr_pa, gpa, tdx_level, hpa, &out);
> 
> kvm_tdx is only used here, can drop the local var.

Will drop it.
-- 
Isaku Yamahata <isaku.yamahata@intel.com>