From: Roman Kisel <romank@xxxxxxxxxxxxxxxxxxx> Sent: Thursday, December 26, 2024 8:46 AM > > On 12/24/2024 8:45 AM, Michael Kelley wrote: > > [...] > > > > > OK, my understanding is that your concern about spec conformance is > > just that Linux should be able to allocate enough input and output space > > for the maximum case, which is 4KiB of input *plus* 4KiB of output. If > > the total amount of input plus output for a particular hypercall is less > > than 4KiB, then there's no conformance problem with having the input > > and output share a page, as long as the "no overlap" rule is observed. > > > Appreciate bearing with me and guiding me towards expressing the intent > clearer :) Yes, the logic chain has been: > > * can't overlap input and output due to TLFS req's => > * need to fix get_vtl() *&&* dom0 uses the output page *&&* VTLs use > the output page => > * let us fix the overlap *&&* make get_vtl() look as get_vp_register() > as this is what it actually is so soon we should be able to have less > code. > > Getting rid of the hypercall output page feels like too much as if the > code base is dovetailed to that and the hypervisor gets a hypercall > whose output is as large as a page (however unlikely that sounds, but > first there was an opinion that 640KiB is plenty, then 32 address lines, > then 48 bits in the PA and 4 level pages, then 57 bits and 5 levels, > ...), we'd need to fix the code or allocate and deallocate on demand. > That tradeoff b/w saving a page and adding special cases makes me lean > to just allocate the page as it is allocated anyway. > > > There's an idea kicking around in my head about a different way to > > handle all this that might be cleaner and less code all-around. If I > > get motivated, I may code it up and see if it really works. If so, > > I'll run it by you to see what you think. > MUCH appreciated!! The complexity appears to be increasing over time, > and it would be incredible to pack all we got into less code without > constraining ourselves too much :) > As I was looking at how hypercall input and output arguments are managed in upstream code and in the OHCL-Linux-Kernel repo, I noticed two things: 1) There's a bug in mshv_vtl_hvcall_call() in the OHCL-Linux-Kernel repo, for which I filed a github issue. [1] 2) hv_vtl_apicid_to_vp_id() also has the overlapping hypercall input and output spec violation. You might want to fix that occurrence as well in this patch set. Michael [1] https://github.com/microsoft/OHCL-Linux-Kernel/issues/33
