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AJvYcCVg9J9lY5vNxicKlkwBqZ1cvkpnIs920vWfdVcklst8iPl+SUVY04YYR3bR3mCgGN1bDHNxn2jll/A=@lists.xenproject.org X-Gm-Message-State: AOJu0YwLN/vMrPDw59BUwCL5rPJjwNI0yX+Q54qI6q9XrHsozcnpx6Nc I5HHdqtvB4Q/zsZo0u0V3CvoKAkBj8lvKebVOz6bMplSkX2Iheg0CTu51Ff5t3I= X-Google-Smtp-Source: AGHT+IH7OPV/oB83+67MnEd133rMpTYtVi4NhaS/AcC+eTlahIUTQNCyuI7Gk7Ggr/Y//VNU39SyPA== X-Received: by 2002:a2e:460a:0:b0:2f2:9f39:3e58 with SMTP id 38308e7fff4ca-2f61054b2c5mr4079511fa.48.1724871411869; Wed, 28 Aug 2024 11:56:51 -0700 (PDT) Message-ID: <17a98dae-51d4-4da0-a35a-1ddf94a06168@citrix.com> Date: Wed, 28 Aug 2024 19:56:49 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 2/4] x86/hvm: Use for_each_set_bit() in hvm_emulate_writeback() To: Jan Beulich Cc: =?UTF-8?Q?Roger_Pau_Monn=C3=A9?= , Xen-devel References: <20240827135746.1908070-1-andrew.cooper3@citrix.com> <20240827135746.1908070-3-andrew.cooper3@citrix.com> <8f34109f-1718-47e5-99c5-a6010d7ebe51@citrix.com> <09742cbe-4c06-49d4-8b26-7ce9076063a1@suse.com> Content-Language: en-GB From: Andrew Cooper Autocrypt: addr=andrew.cooper3@citrix.com; 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charset=UTF-8 Content-Transfer-Encoding: 8bit On 28/08/2024 3:56 pm, Jan Beulich wrote: > On 28.08.2024 16:44, Andrew Cooper wrote: >> On 27/08/2024 5:07 pm, Jan Beulich wrote: >>> On 27.08.2024 15:57, Andrew Cooper wrote: >>>> + for_each_set_bit ( seg, dirty ) >>>> + hvm_set_segment_register(curr, seg, &hvmemul_ctxt->seg_reg[seg]); >>>> + >>>> + hvmemul_ctxt->seg_reg_dirty = 0; >>> Why is this suddenly appearing here? You don't mention it in the description, >>> so it's not clear whether you found a (however minor) issue, or whether >>> that's purely cosmetic (yet then it's still an extra store we could do >>> without). >> Oh, yes.  Nothing anywhere in Xen ever clears these segment dirty bits. > hvm_emulate_init_once()? I meant after emulation.  The value is initialised to 0 at the start of day. > >> I suspect the worst that will go wrong is that we'll waste time >> re-{VMWRITE,memcpy}-ing the segment registers into the VMCS/VMCB, but >> the logic in Xen is definitely not right. > I'm on the edge of asking to do such clearing before emulation, not after > processing the dirty bits. That would then be hvm_emulate_init_per_insn(), > well centralized. Specifically, hvmemul_ctxt should not believe itself to be dirty after a call to hvm_emulate_writeback(), because that's the logic to make the context no-longer-dirty. That said, the more I look at this, the less convinced I am by it.  For a function named writeback(), it's doing a very narrow thing that is not the usual meaning of the term when it comes to pipelines or insn emulation... ~Andrew