lkml.org 
[lkml]   [2022]   [Jan]   [4]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH v4 00/21] AMX Support in KVM
    On Tue, Jan 04, 2022, Paolo Bonzini wrote:
    > On 12/29/21 14:13, Yang Zhong wrote:
    > > Highly appreciate for your review. This version mostly addressed the comments
    > > from Sean. Most comments are adopted except three which are not closed and
    > > need more discussions:
    > >
    > > - Move the entire xfd write emulation code to x86.c. Doing so requires
    > > introducing a new kvm_x86_ops callback to disable msr write bitmap.
    > > According to Paolo's earlier comment he prefers to handle it in vmx.c.
    >
    > Yes, I do.

    No objection, my comments were prior to seeing the patches that manipulated the
    bitmap, e.g. in the earlier patches, having anything in vmx.c is unnecessary.

    > > - Directly check msr_bitmap in update_exception_bitmap() (for
    > > trapping #NM) and vcpu_enter_guest() (for syncing guest xfd after
    > > vm-exit) instead of introducing an extra flag in the last patch. However,
    > > doing so requires another new kvm_x86_ops callback for checking
    > > msr_bitmap since vcpu_enter_guest() is x86 common code. Having an
    > > extra flag sounds simpler here (at least for the initial AMX support).
    > > It does penalize nested guest with one xfd sync per exit, but it's not
    > > worse than a normal guest which initializes xfd but doesn't run
    > > AMX applications at all. Those could be improved afterwards.
    >
    > The thing to do here would be to move
    > MAX_POSSIBLE_PASSTHROUGH_MSRS/MAX_DIRECT_ACCESS_MSRS from VMX/SVM to core
    > code. For now we can keep the flag.
    >
    > > - Disable #NM trap for nested guest. This version still chooses to always
    > > trap #NM (regardless in L1 or L2) as long as xfd write interception is disabled.
    > > In reality #NM is rare if nested guest doesn't intend to run AMX applications
    > > and always-trap is safer than dynamic trap for the basic support in case
    > > of any oversight here.
    >
    > Sean was justifying this with lack of support for nested AMX, but I'm not
    > sure actually what is missing at all. That is, an L1 hypervisor could
    > expose AMX to L2, and then an L2->L0->L2 exit/reentry would have to trap
    > #NM. Otherwise it would miss an XFD_ERR update.

    Ya, I was assuming there was something L0 needed to do to supported nested AMX,
    but as Paolo pointed out there are no VMCS bits, so L0 just needs to correctly
    handle #NM and MSR interceptions according to vmcs12.

    \
     
     \ /
      Last update: 2022-01-04 19:55    [W:4.196 / U:0.036 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site