linux-kernel - Re: [PATCH v3 05/28] drm/i915/gvt: Explicitly check that vGPU is attached before shadowing

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Date:   Mon, 15 May 2023 10:57:07 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Zhi A Wang <zhi.a.wang@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Zhenyu Wang <zhenyuw@...ux.intel.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "intel-gvt-dev@...ts.freedesktop.org" 
        <intel-gvt-dev@...ts.freedesktop.org>,
        "intel-gfx@...ts.freedesktop.org" <intel-gfx@...ts.freedesktop.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Yan Y Zhao <yan.y.zhao@...el.com>,
        Ben Gardon <bgardon@...gle.com>
Subject: Re: [PATCH v3 05/28] drm/i915/gvt: Explicitly check that vGPU is
 attached before shadowing

On Mon, May 15, 2023, Zhi A Wang wrote:
> On 5/13/2023 8:35 AM, Sean Christopherson wrote:
> > Move the check that a vGPU is attacked from is_2MB_gtt_possible() to its
> > sole caller, ppgtt_populate_shadow_entry().  All of the paths in
> > ppgtt_populate_shadow_entry() eventually check for attachment by way of
> > intel_gvt_dma_map_guest_page(), but explicitly checking can avoid
> > unnecessary work and will make it more obvious that a future cleanup of
> > is_2MB_gtt_possible() isn't introducing a bug.
> > 
> 
> It might be better move this check to shadow_ppgtt_mm() which is used
> in both shadow page table creation and pinning path so that the path
> can bail out even earlier when creating a shadow page table but a vGPU
> has not been attached to KVM yet.

Ah, yes, that'll work.  I traced through all of the paths that lead to
ppgtt_populate_shadow_entry(), and shadow_ppgtt_mm() is the only caller that isn't
already gated by INTEL_VGPU_STATUS_ATTACHED or INTEL_VGPU_STATUS_ACTIVE (ACTIVE
is set iff ATTACHED is set).

I'll move the check up to shadow_ppgtt_mm() in the next version.

Thanks!


workload_thread() <= pick_next_workload() => INTEL_VGPU_STATUS_ACTIVE
|
-> dispatch_workload()
   |
   |-> prepare_workload()
       |
       -> intel_vgpu_sync_oos_pages()
       |  |
       |  |-> ppgtt_set_guest_page_sync()
       |      |
       |      |-> sync_oos_page()
       |          |
       |          |-> ppgtt_populate_shadow_entry()
       |
       |-> intel_vgpu_flush_post_shadow()
           |
1:         |-> ppgtt_handle_guest_write_page_table()
               |
               |-> ppgtt_handle_guest_entry_add()
                   |
2:                 | -> ppgtt_populate_spt_by_guest_entry()
                   |    |
                   |    |-> ppgtt_populate_spt()
                   |        |
                   |        |-> ppgtt_populate_shadow_entry()
                   |            |
                   |            |-> ppgtt_populate_spt_by_guest_entry() [see 2]
                   |
                   |-> ppgtt_populate_shadow_entry()


kvmgt_page_track_write()  <= KVM callback => INTEL_VGPU_STATUS_ATTACHED
|
|-> intel_vgpu_page_track_handler()
    |
    |-> ppgtt_write_protection_handler()
        |
        |-> ppgtt_handle_guest_write_page_table_bytes()
            |
            |-> ppgtt_handle_guest_write_page_table() [see 1]