[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [PATCH 1/7] x86/HVM: drop stdvga's "cache" struct member
On 10.09.2024 18:44, Andrew Cooper wrote: > On 10/09/2024 3:39 pm, Jan Beulich wrote: >> As of 68e1183411be ("libxc: introduce a xc_dom_arch for hvm-3.0-x86_32 >> guests") caching mode is disabled for HVM domains from start-of-day, due >> the disabling being unconditional in hvm/save.c:arch_hvm_load(). With >> that the field is useless, and can be dropped. Drop the helper functions >> manipulating / checking as well right away, but leave the use sites of >> stdvga_cache_is_enabled() with the hard-coded result the function would >> have produced, to aid validation of subsequent dropping of further code. >> >> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx> > > This only applies to VMs constructed with libxenguest, which isn't all > VMs. But, it's probably close enough to ~100% of VMs to count. > > Personally I think it would be clearer to say "Since 68e1183411be, HVM > guests are built using XEN_DOMCTL_sethvmcontext, which intentionally > disables stdvga caching in arch_hvm_load()". I'll derive something intermediate. I don't think it was intentional that caching got disabled this way. It's just that no-one noticed or paid attention. > As a minor tangent, this is yet another casualty of nothing being wired > into the migration stream. Rightly or wrongly, that mindset has caused > an immense amount of problems in Xen. Indeed. > How does this interact with X86_EMU_VGA? > > stdvga_init() is called unconditionally for HVM domains, exiting early > for !EMU_VGA (skipping the pointless re-zero of the structure). > Nevertheless, the cache is UNINITIALISED for all configurations at this > point. > > And we won't hit the stdvga_try_cache_enable() calls in any of > stdvga_{outb,mem_{write,accept}}() prior to XEN_DOMCTL_sethvmcontext, > which will unconditionally set DISABLED. > > So yes, I think this is for-all-intents-and-purposes dead logic. > > Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx> Thanks. Jan
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