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AJvYcCW4YqWYpQp8zN+G7DTfGud0Ik0bQp+eUPJzhQaj6dZPq4Sccx6/xMCvyoCy1sajx5ZFy4e+VeqY5dA=@lists.xenproject.org X-Gm-Message-State: AOJu0YxIqZ/Ft1eAn6Ha62TKNWQTHLbtwsdvNk1eyZNfcBYnjqk38H2M ZZg6l+o3oXwmZuEQ4b19ognuV0+At8Vm5fJ112A7NA4v8CXePCeC51kmSY+3pNU= X-Google-Smtp-Source: AGHT+IG2ofPpTNYMcB1E84LvSFh39mC/64iRd/LXo4ANMC3blm/Q9OctNkJY1KxoFfVELdrruRXOCA== X-Received: by 2002:a05:6830:720d:b0:70f:3dd0:316c with SMTP id 46e09a7af769-70f3dd03473mr11658646a34.27.1724856264272; Wed, 28 Aug 2024 07:44:24 -0700 (PDT) Message-ID: <8f34109f-1718-47e5-99c5-a6010d7ebe51@citrix.com> Date: Wed, 28 Aug 2024 15:44:21 +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> 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 27/08/2024 5:07 pm, Jan Beulich wrote: > On 27.08.2024 15:57, Andrew Cooper wrote: >> ... which is more consise than the opencoded form. >> >> Also, for production VMs, ~100% of emulations are simple MOVs, so it is likely >> that there are no segments to write back. >> >> Furthermore, now that find_{first,next}_bit() are no longer in use, the >> seg_reg_{accessed,dirty} fields aren't forced to be unsigned long, although >> they do need to remain unsigned int because of __set_bit() elsewhere. >> >> No practical change. >> >> Signed-off-by: Andrew Cooper >> --- >> CC: Jan Beulich >> CC: Roger Pau Monné >> >> Pulling current out into curr is good for code generation. When using current >> in the loop, GCC can't retain the calculation across the call to >> hvm_set_segment_register() and is forced to re-read from the cpu_info block. >> >> However, if curr is initialised, it's calculated even in the likely path... > That's a little odd, as I don't think I can spot what would force the compiler > into doing so. As a wild guess, ... > >> --- a/xen/arch/x86/hvm/emulate.c >> +++ b/xen/arch/x86/hvm/emulate.c >> @@ -2908,18 +2908,18 @@ void hvm_emulate_init_per_insn( >> void hvm_emulate_writeback( >> struct hvm_emulate_ctxt *hvmemul_ctxt) >> { >> - enum x86_segment seg; >> + struct vcpu *curr; >> + unsigned int dirty = hvmemul_ctxt->seg_reg_dirty; > ... is the order of these two possibly relevant? Yet of course it's not the > end of the world whichever way it's done. My general conclusion is that GCC doesn't try very hard to optimise likely() early exits. I suspect there's some reasonably low hanging fruit to be found there. > >> - seg = find_first_bit(&hvmemul_ctxt->seg_reg_dirty, >> - ARRAY_SIZE(hvmemul_ctxt->seg_reg)); >> + if ( likely(!dirty) ) >> + return; >> >> - while ( seg < ARRAY_SIZE(hvmemul_ctxt->seg_reg) ) >> - { >> - hvm_set_segment_register(current, seg, &hvmemul_ctxt->seg_reg[seg]); >> - seg = find_next_bit(&hvmemul_ctxt->seg_reg_dirty, >> - ARRAY_SIZE(hvmemul_ctxt->seg_reg), >> - seg+1); >> - } >> + curr = current; >> + >> + 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. 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. ~Andrew