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AJvYcCUrup3XW7uYPLigoZyNWbrc9i2wr6hBg4/xQNFJSTTpTtpyQkCFqkb2aOzdlxfqw3UBditNBl+4J9I=@lists.xenproject.org X-Gm-Message-State: AOJu0Yze7wbI8ZGgjSwq+X4x23AbnRWr5l1eexDcQpS+hYUOpyzSgVzl zvs0g8oHYxw5vesR6efj3U9FJcZVkKSmrZJgSyRa9lqD7/dUdcayJ401njF3ro8= X-Google-Smtp-Source: AGHT+IEcdmxGCSfm272Ko8HtpRoWYJdkGIxzeQjM9KMmnHs5/Ryn2EmyDypdbk4gqCDqoT41unT6Sg== X-Received: by 2002:a05:6402:5253:b0:5cf:9cd1:1f77 with SMTP id 4fb4d7f45d1cf-5cff4c52b03mr3724929a12.8.1732188381144; Thu, 21 Nov 2024 03:26:21 -0800 (PST) Message-ID: <03053e3b-c101-4cc2-9f59-604e84f92ea0@citrix.com> Date: Thu, 21 Nov 2024 11:26:19 +0000 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] x86/pv: limit GDT and LDT mappings areas to max number of vCPUs To: Roger Pau Monne , xen-devel@lists.xenproject.org Cc: Jan Beulich References: <20241121111218.50984-1-roger.pau@citrix.com> Content-Language: en-GB From: Andrew Cooper Autocrypt: addr=andrew.cooper3@citrix.com; keydata= xsFNBFLhNn8BEADVhE+Hb8i0GV6mihnnr/uiQQdPF8kUoFzCOPXkf7jQ5sLYeJa0cQi6Penp VtiFYznTairnVsN5J+ujSTIb+OlMSJUWV4opS7WVNnxHbFTPYZVQ3erv7NKc2iVizCRZ2Kxn srM1oPXWRic8BIAdYOKOloF2300SL/bIpeD+x7h3w9B/qez7nOin5NzkxgFoaUeIal12pXSR Q354FKFoy6Vh96gc4VRqte3jw8mPuJQpfws+Pb+swvSf/i1q1+1I4jsRQQh2m6OTADHIqg2E ofTYAEh7R5HfPx0EXoEDMdRjOeKn8+vvkAwhviWXTHlG3R1QkbE5M/oywnZ83udJmi+lxjJ5 YhQ5IzomvJ16H0Bq+TLyVLO/VRksp1VR9HxCzItLNCS8PdpYYz5TC204ViycobYU65WMpzWe LFAGn8jSS25XIpqv0Y9k87dLbctKKA14Ifw2kq5OIVu2FuX+3i446JOa2vpCI9GcjCzi3oHV e00bzYiHMIl0FICrNJU0Kjho8pdo0m2uxkn6SYEpogAy9pnatUlO+erL4LqFUO7GXSdBRbw5 gNt25XTLdSFuZtMxkY3tq8MFss5QnjhehCVPEpE6y9ZjI4XB8ad1G4oBHVGK5LMsvg22PfMJ ISWFSHoF/B5+lHkCKWkFxZ0gZn33ju5n6/FOdEx4B8cMJt+cWwARAQABzSlBbmRyZXcgQ29v cGVyIDxhbmRyZXcuY29vcGVyM0BjaXRyaXguY29tPsLBegQTAQgAJAIbAwULCQgHAwUVCgkI CwUWAgMBAAIeAQIXgAUCWKD95wIZAQAKCRBlw/kGpdefoHbdD/9AIoR3k6fKl+RFiFpyAhvO 59ttDFI7nIAnlYngev2XUR3acFElJATHSDO0ju+hqWqAb8kVijXLops0gOfqt3VPZq9cuHlh IMDquatGLzAadfFx2eQYIYT+FYuMoPZy/aTUazmJIDVxP7L383grjIkn+7tAv+qeDfE+txL4 SAm1UHNvmdfgL2/lcmL3xRh7sub3nJilM93RWX1Pe5LBSDXO45uzCGEdst6uSlzYR/MEr+5Z JQQ32JV64zwvf/aKaagSQSQMYNX9JFgfZ3TKWC1KJQbX5ssoX/5hNLqxMcZV3TN7kU8I3kjK mPec9+1nECOjjJSO/h4P0sBZyIUGfguwzhEeGf4sMCuSEM4xjCnwiBwftR17sr0spYcOpqET ZGcAmyYcNjy6CYadNCnfR40vhhWuCfNCBzWnUW0lFoo12wb0YnzoOLjvfD6OL3JjIUJNOmJy RCsJ5IA/Iz33RhSVRmROu+TztwuThClw63g7+hoyewv7BemKyuU6FTVhjjW+XUWmS/FzknSi dAG+insr0746cTPpSkGl3KAXeWDGJzve7/SBBfyznWCMGaf8E2P1oOdIZRxHgWj0zNr1+ooF /PzgLPiCI4OMUttTlEKChgbUTQ+5o0P080JojqfXwbPAyumbaYcQNiH1/xYbJdOFSiBv9rpt TQTBLzDKXok86M7BTQRS4TZ/ARAAkgqudHsp+hd82UVkvgnlqZjzz2vyrYfz7bkPtXaGb9H4 Rfo7mQsEQavEBdWWjbga6eMnDqtu+FC+qeTGYebToxEyp2lKDSoAsvt8w82tIlP/EbmRbDVn 7bhjBlfRcFjVYw8uVDPptT0TV47vpoCVkTwcyb6OltJrvg/QzV9f07DJswuda1JH3/qvYu0p vjPnYvCq4NsqY2XSdAJ02HrdYPFtNyPEntu1n1KK+gJrstjtw7KsZ4ygXYrsm/oCBiVW/OgU g/XIlGErkrxe4vQvJyVwg6YH653YTX5hLLUEL1NS4TCo47RP+wi6y+TnuAL36UtK/uFyEuPy wwrDVcC4cIFhYSfsO0BumEI65yu7a8aHbGfq2lW251UcoU48Z27ZUUZd2Dr6O/n8poQHbaTd 6bJJSjzGGHZVbRP9UQ3lkmkmc0+XCHmj5WhwNNYjgbbmML7y0fsJT5RgvefAIFfHBg7fTY/i kBEimoUsTEQz+N4hbKwo1hULfVxDJStE4sbPhjbsPCrlXf6W9CxSyQ0qmZ2bXsLQYRj2xqd1 bpA+1o1j2N4/au1R/uSiUFjewJdT/LX1EklKDcQwpk06Af/N7VZtSfEJeRV04unbsKVXWZAk uAJyDDKN99ziC0Wz5kcPyVD1HNf8bgaqGDzrv3TfYjwqayRFcMf7xJaL9xXedMcAEQEAAcLB XwQYAQgACQUCUuE2fwIbDAAKCRBlw/kGpdefoG4XEACD1Qf/er8EA7g23HMxYWd3FXHThrVQ HgiGdk5Yh632vjOm9L4sd/GCEACVQKjsu98e8o3ysitFlznEns5EAAXEbITrgKWXDDUWGYxd pnjj2u+GkVdsOAGk0kxczX6s+VRBhpbBI2PWnOsRJgU2n10PZ3mZD4Xu9kU2IXYmuW+e5KCA vTArRUdCrAtIa1k01sPipPPw6dfxx2e5asy21YOytzxuWFfJTGnVxZZSCyLUO83sh6OZhJkk b9rxL9wPmpN/t2IPaEKoAc0FTQZS36wAMOXkBh24PQ9gaLJvfPKpNzGD8XWR5HHF0NLIJhgg 4ZlEXQ2fVp3XrtocHqhu4UZR4koCijgB8sB7Tb0GCpwK+C4UePdFLfhKyRdSXuvY3AHJd4CP 4JzW0Bzq/WXY3XMOzUTYApGQpnUpdOmuQSfpV9MQO+/jo7r6yPbxT7CwRS5dcQPzUiuHLK9i nvjREdh84qycnx0/6dDroYhp0DFv4udxuAvt1h4wGwTPRQZerSm4xaYegEFusyhbZrI0U9tJ B8WrhBLXDiYlyJT6zOV2yZFuW47VrLsjYnHwn27hmxTC/7tvG3euCklmkn9Sl9IAKFu29RSo d5bD8kMSCYsTqtTfT6W4A3qHGvIDta3ptLYpIAOD2sY3GYq2nf3Bbzx81wZK14JdDDHUX2Rs 6+ahAA== In-Reply-To: <20241121111218.50984-1-roger.pau@citrix.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 21/11/2024 11:12 am, Roger Pau Monne wrote: > The allocation of the paging structures in the per-domain area for mapping the > guest GDT and LDT can be limited to the maximum number of vCPUs the guest can > have. The maximum number of vCPUs is available at domain creation since commit > 4737fa52ce86. > > Limiting to the actual number of vCPUs avoids wasting memory for paging > structures that will never be used. Current logic unconditionally uses 513 > pages, one page for the L3, plus 512 L1 pages. For guests with equal or less > than 16 vCPUs only 2 pages are used (each guest vCPU GDT/LDT can only consume > 32 L1 slots). > > No functional change intended, all possible domain vCPUs should have the GDT > and LDT paging structures allocated and setup at domain creation, just like > before the change. > > Signed-off-by: Roger Pau Monné Ooh, that's a optimisation I'd not considered when doing the work. But, is it really only the the LDT/GDT area which can benefit from this?  The XLAT area seems like another good candidate. > --- > xen/arch/x86/pv/domain.c | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/xen/arch/x86/pv/domain.c b/xen/arch/x86/pv/domain.c > index d5a8564c1cbe..e861e3ce71d9 100644 > --- a/xen/arch/x86/pv/domain.c > +++ b/xen/arch/x86/pv/domain.c > @@ -346,7 +346,7 @@ void pv_domain_destroy(struct domain *d) > pv_l1tf_domain_destroy(d); > > destroy_perdomain_mapping(d, GDT_LDT_VIRT_START, > - GDT_LDT_MBYTES << (20 - PAGE_SHIFT)); > + d->max_vcpus << GDT_LDT_VCPU_SHIFT); Probably not for this patch, but, should we really be passing in a size here? Don't we just want to tear down everything in the relevant slot? > > XFREE(d->arch.pv.cpuidmasks); > > @@ -377,7 +377,7 @@ int pv_domain_initialise(struct domain *d) > goto fail; > > rc = create_perdomain_mapping(d, GDT_LDT_VIRT_START, > - GDT_LDT_MBYTES << (20 - PAGE_SHIFT), > + d->max_vcpus << GDT_LDT_VCPU_SHIFT, > NULL, NULL); I'd suggest putting a note here saying that the maximum bound for PV vCPUs is governed by GDT_LDT_MBYTES. Or alternatively, we could have create_perdomain_mapping() fail if it tries to allocate more than one slot's worth of mappings?   It feels like an acceptable safety net. ~Andrew